fix(sessions): bypass UserDraft inside session panes + sessionUnread crash

After merging main's UserDraft PR (#9121) into the sessions branch, two
integration issues surfaced:

1. AppEditor.svelte calls `UserDraft.use<App>('app', path)` at the
   component level — keyed by ($workspaceStore, 'app', path). Sessions
   that haven't materialized a fork yet stay at the user's main
   workspace, so a session targeting an app at the same path as a
   regular /apps/edit tab shared the same LS key. The session would
   read the regular tab's autosave and write its fork-edits back over
   it.

   Gate UserDraft.use on `!getContext('aiChatManager')` — sessions
   inject the manager via setContext, so inside a session pane the
   handle is `undefined`, stateApp falls through to the `app` prop
   the session loaded, and the auto-save $effect bails. Same gate on
   the four UserDraft.remove call sites in AppEditorHeader and
   RawAppEditorHeader so save/deploy from a session pane doesn't wipe
   the LS draft of a non-session tab at the same path.

2. sessionUnread.svelte.ts called useLocalStorageValue at module
   scope. Main's PR added a deep-mutation $effect inside that helper,
   which now requires component-initialization context — every page
   crashed at import time with `Svelte error: effect_orphan`.
   Replaced with a plain module-level $state + manual localStorage
   persist; same reactivity contract for callers.

3. ScriptEditorView.svelte was passing a `replaceStateFn` prop that
   ScriptBuilder dropped on main. Removed.

Verified end-to-end with Playwright:
- /flows/edit/{path} regression: UserDraft handle still created, no
  console errors
- /sessions loads, sessionUnread doesn't crash
- Session targeting non-raw app `u/admin/userdraft_collision_test`
  displays the fork content (FORK_ONLY_MARKER) even with an LS
  poison at `userdraft/w/local/app/{path}` containing a
  POISONED_BY_REGULAR_TAB_AUTOSAVE marker; poison remains untouched
  after the session loads and renders

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/lib/components/apps/editor/AppEditor.svelte

@@ -3,7 +3,7 @@
 
 	const bubble = createBubbler()
 	import SplitPanesWrapper from '$lib/components/splitPanes/SplitPanesWrapper.svelte'
-	import { onMount, setContext, untrack } from 'svelte'
+	import { getContext, onMount, setContext, untrack } from 'svelte'
 	import { twMerge } from 'tailwind-merge'
 
 	import { Pane, Splitpanes } from 'svelte-splitpanes'
@@ -85,15 +85,23 @@
 
 	migrateApp(untrack(() => app))
 
+	// Inside a session pane the AIChatManager is injected via context. Sessions
+	// have their own state machinery (sessionRuntime + per-fork backend), and
+	// the user-facing $workspaceStore stays on the main workspace even when
+	// the session is editing in a fork — so a UserDraft handle here would
+	// share its LS key with the regular /apps/edit route and clobber both
+	// sides' autosaves. Skip UserDraft entirely in that case.
+	const inSessionPane = !!getContext('aiChatManager')
+
 	const appDraftPath = newApp ? '' : (path ?? '')
-	const appDraftHandle = UserDraft.use<App>('app', appDraftPath)
+	const appDraftHandle = inSessionPane ? undefined : UserDraft.use<App>('app', appDraftPath)
 	// Prefer the persisted autosave over the prop when both exist (e.g.
 	// /apps/add reload: the route always initializes `app` to an empty
 	// template, but the user's last session is sitting in LS under the
 	// empty-path entry). The route is responsible for wiping the entry
 	// (`UserDraft.remove`) when it wants to force a fresh start —
 	// `?nodraft=true`, template/hub loads, etc.
-	const stateApp = $state(untrack(() => appDraftHandle.draft ?? app))
+	const stateApp = $state(untrack(() => appDraftHandle?.draft ?? app))
 	const appStore = writable<App>(stateApp)
 	// Captured once on mount: the load-time revs are only used as the
 	// seed meta on the very first persist of this entry. After that the
@@ -113,6 +121,7 @@
 	let firstMirror = true
 	$effect(() => {
 		readFieldsRecursively(stateApp)
+		if (!appDraftHandle) return
 		untrack(() => {
 			// Resolve the meta to attach BEFORE the wipe — the wipe clears
 			// in-memory meta and would otherwise force-seed `initialRevs`

frontend/src/lib/components/apps/editor/AppEditorHeader.svelte

@@ -172,6 +172,11 @@
 	const { history, jobsDrawerOpen, refreshComponents } =
 		getContext<AppEditorContext>('AppEditorContext')
 
+	// Sessions inject an AIChatManager via context; AppEditor skips its
+	// UserDraft handle in that case, so the cleanup calls here must skip too
+	// (otherwise we'd wipe a non-session tab's autosave at the same path).
+	const inSessionPane = !!getContext('aiChatManager')
+
 	const loading = $state({
 		publish: false,
 		save: false,
@@ -231,7 +236,7 @@
 			}
 			closeSaveDrawer()
 			sendUserToast('App deployed successfully')
-			UserDraft.remove('app', path)
+			if (!inSessionPane) UserDraft.remove('app', path)
 			onSavedNewAppPath?.(path)
 		} catch (e) {
 			sendUserToast('Error creating app', e)
@@ -331,7 +336,7 @@
 
 		closeSaveDrawer()
 		sendUserToast('App deployed successfully')
-		UserDraft.remove('app', $appPath)
+		if (!inSessionPane) UserDraft.remove('app', $appPath)
 		if ($appPath !== npath) {
 			onSavedNewAppPath?.(npath)
 		}
@@ -407,7 +412,7 @@
 			// The initial draft was promoted to a real path on the backend —
 			// drop the autosave keyed on the prior (possibly empty) path so
 			// a future "+ App" click opens on a clean slate.
-			UserDraft.remove('app', $appPath)
+			if (!inSessionPane) UserDraft.remove('app', $appPath)
 			onSavedNewAppPath?.(newEditedPath)
 		} catch (e) {
 			sendUserToast('Error saving initial draft', e)
@@ -498,7 +503,7 @@
 			}
 
 			sendUserToast('Draft saved')
-			UserDraft.remove('app', path)
+			if (!inSessionPane) UserDraft.remove('app', path)
 			loading.saveDraft = false
 			if (newApp || savedApp.draft_only) {
 				onSavedNewAppPath?.(newEditedPath || path)

frontend/src/lib/components/raw_apps/RawAppEditorHeader.svelte

@@ -269,7 +269,7 @@
 			}
 			closeSaveDrawer()
 			sendUserToast('App deployed successfully')
-			UserDraft.remove('raw_app', path)
+			if (!inSessionPane) UserDraft.remove('raw_app', path)
 			dispatch('savedNewAppPath', path)
 		} catch (e) {
 			sendUserToast(`Error creating app: ${e.body ?? e.message}`, true)
@@ -380,7 +380,7 @@
 
 		closeSaveDrawer()
 		sendUserToast('App deployed successfully')
-		UserDraft.remove('raw_app', appPath)
+		if (!inSessionPane) UserDraft.remove('raw_app', appPath)
 		if (appPath !== npath) {
 			dispatch('savedNewAppPath', npath)
 		}
@@ -462,7 +462,7 @@
 			// The initial draft was promoted to a real path on the backend —
 			// drop the autosave keyed on the prior (possibly empty) path so
 			// a future "+ App" click opens on a clean slate.
-			UserDraft.remove('raw_app', appPath)
+			if (!inSessionPane) UserDraft.remove('raw_app', appPath)
 			dispatch('savedNewAppPath', newEditedPath)
 		} catch (e) {
 			sendUserToast(`Error saving initial draft: ${e.body ?? e.message}`, true)
@@ -563,7 +563,7 @@
 			}
 
 			sendUserToast('Draft saved')
-			UserDraft.remove('raw_app', path)
+			if (!inSessionPane) UserDraft.remove('raw_app', path)
 			loading.saveDraft = false
 			if (newApp || savedApp.draft_only) {
 				dispatch('savedNewAppPath', newEditedPath || path)

frontend/src/lib/components/sessions/ScriptEditorView.svelte

@@ -112,7 +112,6 @@
 		initialPath={path}
 		fullyLoaded={!runtime.loadingScript}
 		disableHistoryChange={true}
-		replaceStateFn={() => {}}
 		{diffDrawer}
 		{onNavigate}
 		{initialTestPanelCollapsed}

frontend/src/lib/components/sessions/sessionUnread.svelte.ts

@@ -1,40 +1,60 @@
-import { useLocalStorageValue } from '$lib/svelte5Utils.svelte'
 import type { SessionRuntime } from './sessionRuntime.svelte'
 
 // Per-user, per-session "last seen" marker — count of displayMessages the
 // last time the user was actually on that session's page. Compared against
 // the runtime's current message count to derive an unread badge.
 //
-// Stored as a single localStorage entry holding Record<sessionId, count>
-// to avoid scattering keys across the namespace and to make
-// useLocalStorageValue's reactivity cover all sessions at once.
-const lastSeenStore = useLocalStorageValue<Record<string, number>>(
-	'windmill_sessions_last_seen_counts',
-	{}
-)
+// Stored as a single localStorage entry holding Record<sessionId, count>.
+// Module-level $state for cross-session reactivity; can't use
+// `useLocalStorageValue` here because its internal $effect requires a
+// component-initialization context (we run at import time).
+const LS_KEY = 'windmill_sessions_last_seen_counts'
+
+function readInitial(): Record<string, number> {
+	if (typeof window === 'undefined') return {}
+	try {
+		const raw = localStorage.getItem(LS_KEY)
+		return raw ? JSON.parse(raw) : {}
+	} catch {
+		return {}
+	}
+}
+
+const lastSeen = $state<{ val: Record<string, number> }>({ val: readInitial() })
+
+function persist(): void {
+	if (typeof window === 'undefined') return
+	try {
+		localStorage.setItem(LS_KEY, JSON.stringify(lastSeen.val))
+	} catch (e) {
+		console.error('sessionUnread: localStorage write failed', e)
+	}
+}
 
 // Mark the session as seen up to `count` messages. No-op when already at
 // or past that count (idempotent — call freely from $effects).
 export function markSessionSeen(sessionId: string, count: number) {
-	const current = lastSeenStore.val[sessionId] ?? 0
+	const current = lastSeen.val[sessionId] ?? 0
 	if (current >= count) return
-	lastSeenStore.val = { ...lastSeenStore.val, [sessionId]: count }
+	lastSeen.val = { ...lastSeen.val, [sessionId]: count }
+	persist()
 }
 
 // Drop the session entry entirely (used on delete so we don't leak
 // stale ids into localStorage indefinitely).
 export function forgetSessionSeen(sessionId: string) {
-	if (!(sessionId in lastSeenStore.val)) return
-	const next = { ...lastSeenStore.val }
+	if (!(sessionId in lastSeen.val)) return
+	const next = { ...lastSeen.val }
 	delete next[sessionId]
-	lastSeenStore.val = next
+	lastSeen.val = next
+	persist()
 }
 
 // Number of unread messages for a session. Undefined / unloaded runtime
 // returns 0 — until messages are hydrated we don't know what's new.
 export function unreadCountFor(sessionId: string, runtime: SessionRuntime | undefined): number {
 	if (!runtime) return 0
-	const seen = lastSeenStore.val[sessionId] ?? 0
+	const seen = lastSeen.val[sessionId] ?? 0
 	const total = runtime.manager.displayMessages.length
 	return Math.max(0, total - seen)
 }