feat(script-editor): wire initialTestPanelCollapsed through ScriptBuilder

The `initialTestPanelCollapsed` prop was already declared on
`ScriptBuilderProps` (used by the session preview to start the editor
with the run/test pane closed) but never destructured in
`ScriptBuilder.svelte`, so the value silently dropped on the floor
and the test pane always opened.

- `ScriptBuilder.svelte` — destructure the prop and forward it to
  `<ScriptEditor>`.
- `ScriptEditor.svelte` — accept the prop and seed `rawTestPanelSize`
  to 0 when true, while keeping `storedTestPanelSize` at the default
  30 so the user's first toggle expands the pane to a sensible width
  rather than 0.

Regular `/scripts/edit/...` doesn't pass the prop → default `false`
→ panel still opens by default.

frontend/src/lib/components/ScriptBuilder.svelte

@@ -134,7 +134,8 @@
 		onSaveDraftError,
 		onSaveDraft,
 		onNavigate,
-		disableAi
+		disableAi,
+		initialTestPanelCollapsed = false
 	}: ScriptBuilderProps = $props()
 
 	export function getInitialAndModifiedValues(): SavedAndModifiedValue {
@@ -2091,6 +2092,7 @@
 			bind:assets={script.assets}
 			bind:modules={script.modules}
 			enablePreprocessorSnippet
+			{initialTestPanelCollapsed}
 		/>
 	</div>
 {:else}

frontend/src/lib/components/ScriptEditor.svelte

@@ -159,6 +159,11 @@
 		modules?: { [key: string]: ScriptModule } | null
 		editorBarRight?: import('svelte').Snippet
 		enablePreprocessorSnippet?: boolean
+		// When true the right-hand test/run pane mounts collapsed. The user
+		// can still expand it via `toggleTestPanel`. Defaults to false so the
+		// regular /scripts/edit route keeps its current open-by-default UX;
+		// the session preview opts in to save vertical real estate.
+		initialTestPanelCollapsed?: boolean
 	}
 
 	let {
@@ -192,7 +197,8 @@
 		assets = $bindable(),
 		modules = $bindable(undefined),
 		editorBarRight,
-		enablePreprocessorSnippet = false
+		enablePreprocessorSnippet = false,
+		initialTestPanelCollapsed = false
 	}: Props = $props()
 
 	let initialArgs = structuredClone($state.snapshot(args))
@@ -1359,8 +1365,11 @@
 	// dynamic minimum below — so when the editor shrinks, the displayed test
 	// pane grows to honor the new minimum without needing an effect. The code
 	// pane's size is purely derived from it (100 - test).
-	let rawTestPanelSize = $state(30)
-	let storedTestPanelSize = untrack(() => rawTestPanelSize)
+	// `initialTestPanelCollapsed` seeds the raw value at 0 (collapsed) while
+	// keeping the "remembered" size at 30, so the user's first toggle expands
+	// the pane to a sensible width rather than 0.
+	let rawTestPanelSize = $state(untrack(() => (initialTestPanelCollapsed ? 0 : 30)))
+	let storedTestPanelSize = 30
 	const testPanelSize = $derived(
 		rawTestPanelSize === 0 ? 0 : Math.max(rawTestPanelSize, testPaneMinPercent)
 	)