feat(studio): storyboard frame contact-sheet grid (#1530)

Third PR in the Studio storyboarding stack. Renders the frames as a live
contact sheet inside the storyboard view.

- StoryboardGrid: ordered, responsive grid of frame tiles.
- StoryboardFrameTile: number badge, scaled non-interactive live preview
  iframe (via /api/projects/:id/preview/comp/<src>), title, duration,
  transition, and a status chip (outline / built / animated).
- Frames that are outline-only or whose src is missing render an explicit
  placeholder instead of an iframe.
- StoryboardView swaps its placeholder for the real grid.

With PR1-PR3 the storyboard view is end-to-end viewable against the
storyboard-sample fixture for UI/UX feedback.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/studio/src/hooks/useStoryboard.ts

@@ -12,6 +12,13 @@ export interface StoryboardFrameView extends StoryboardFrame {
   srcExists: boolean;
 }
 
+/** The companion narration script (SCRIPT.md), when present alongside the storyboard. */
+export interface StoryboardScript {
+  exists: boolean;
+  path: string;
+  content: string;
+}
+
 /** Shape of `GET /api/projects/:id/storyboard`. */
 export interface StoryboardResponse {
   exists: boolean;
@@ -19,6 +26,7 @@ export interface StoryboardResponse {
   globals: StoryboardGlobals;
   frames: StoryboardFrameView[];
   warnings: StoryboardWarning[];
+  script?: StoryboardScript;
 }
 
 export interface UseStoryboardResult {