fix(studio): storyboard polish — a11y, preview, and edit-race fixes (#1544)

Batched non-blocking review nits from the storyboard stack (#1528–#1532):

- StoryboardGrid: responsive auto-fill grid instead of fixed-360 tiles
- FramePoster: reset failed state when the poster target changes (stale-error fix)
- StoryboardFrameTile: status-chip aria-label
- StoryboardSourceEditor: marked({async:false}); save() in-flight guard;
  immediate first preview paint; [&_img] prose; scoped link-hardening
  (rel=noopener noreferrer + target=_blank) in the sanitizer
- StoryboardLoaded: memoize sourceFiles on data.script.path/.exists, not the object ref
- StoryboardFrameFocus: applyEdit in-flight guard; aria-pressed on status buttons;
  ←/→/Esc keyboard navigation
- ViewModeContext: correct the popstate/replaceState doc-drift

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

packages/studio/src/contexts/ViewModeContext.tsx

@@ -53,7 +53,12 @@ export function useViewModeState(enabled: boolean): ViewModeValue {
     enabled ? readViewModeFromUrl() : "timeline",
   );
 
-  // Reflect back/forward navigation and agent-driven URL changes.
+  // Reflect genuine browser back/forward between history entries with a different
+  // `?view=`. Note: our own writes use `replaceState` (below), which does NOT fire
+  // `popstate`, so this listener never sees them — `setViewMode` updates state directly.
+  // An agent deep-links by doing a full navigation to `?view=storyboard` (picked up by
+  // the mount-time read); a scripted `pushState`/`replaceState` to `?view=` would not be
+  // reflected here, by design.
   useEffect(() => {
     if (!enabled) return;
     const onPopState = () => setMode(readViewModeFromUrl());