refactor(dashboard): extract the board's client-host read

The merge of main's label bounding pushed build-dashboard-snapshot.ts to
302 lines. The client's own platform facts are a distinct concept from the
pty host each card keys against, so they move out rather than earn a
max-lines bypass.

src/renderer/src/components/dashboard/build-dashboard-snapshot.ts

@@ -14,6 +14,7 @@ import {
   resolveDashboardCardTerminalInput,
   type DashboardCardTerminalInputState
 } from './dashboard-card-terminal-input'
+import { readDashboardClientHost } from './dashboard-client-host'
 import { getAgentRowConversationName } from '../../../../shared/agent-row-conversation-name'
 import { migrationUnsupportedToAgentStatusEntry } from '@/lib/migration-unsupported-agent-entry'
 import { applyAgentRowLineage } from './agent-row-lineage'
@@ -61,14 +62,6 @@ export type DashboardSnapshotState = Pick<
   DashboardCardContextState &
   Partial<DashboardCardTerminalInputState>
 
-function readClientOsRelease(): string | undefined {
-  try {
-    return window.api?.platform?.get?.()?.osRelease
-  } catch {
-    return undefined
-  }
-}
-
 function bucketForState(state: DashboardAgentRow['state']): DashboardBucket {
   switch (state) {
     case 'working':
@@ -136,13 +129,7 @@ export function buildDashboardSnapshot(
   options: { includeCardDetails?: boolean; includeFilterOptions?: boolean } = {}
 ): DashboardSnapshot {
   const cards: DashboardCard[] = []
-  const clientUserAgent = typeof navigator === 'undefined' ? '' : navigator.userAgent
-  const clientPlatform = clientUserAgent.includes('Mac')
-    ? 'darwin'
-    : clientUserAgent.includes('Windows')
-      ? 'win32'
-      : 'linux'
-  const clientOsRelease = readClientOsRelease()
+  const clientHost = readDashboardClientHost()
   const repoIconsByRepoId: Record<string, RepoIcon | null> = {}
   const includeCardDetails = options.includeCardDetails !== false
   const generatedTitlesEnabled = state.settings?.tabAutoGenerateTitle === true
@@ -294,9 +281,9 @@ export function buildDashboardSnapshot(
               cwd: row.tab.startupCwd ?? worktree.path,
               shellOverride: row.tab.shellOverride,
               launchAgent: row.tab.launchAgent,
-              clientPlatform,
-              userAgent: clientUserAgent,
-              osRelease: clientOsRelease
+              clientPlatform: clientHost.platform,
+              userAgent: clientHost.userAgent,
+              osRelease: clientHost.osRelease
             })
           : null
       // Only repos that actually contribute a card ship their icon.

src/renderer/src/components/dashboard/dashboard-client-host.ts

@@ -0,0 +1,32 @@
+/**
+ * The facts about the machine *rendering* the board, as opposed to the machine
+ * executing a pty. A card's preview terminal needs both: byte encodings follow
+ * the pty's host, while xterm's own ConPTY wrap-marker compat follows the client.
+ */
+export type DashboardClientHost = {
+  platform: NodeJS.Platform
+  userAgent: string
+  osRelease: string | undefined
+}
+
+export function readDashboardClientHost(): DashboardClientHost {
+  const userAgent = typeof navigator === 'undefined' ? '' : navigator.userAgent
+  return {
+    platform: userAgent.includes('Mac')
+      ? 'darwin'
+      : userAgent.includes('Windows')
+        ? 'win32'
+        : 'linux',
+    userAgent,
+    // Why: absent in the pop-out renderer and in tests, where no preload rides along.
+    osRelease: readClientOsRelease()
+  }
+}
+
+function readClientOsRelease(): string | undefined {
+  try {
+    return window.api?.platform?.get?.()?.osRelease
+  } catch {
+    return undefined
+  }
+}