annotations v2: redesign

Reskin to v2 surface/accent tokens + JetBrains Mono headings to match
_docs/ui_design/v2/plugin/annotations.html. Add scrubber with class-colored
annotation marks, canvas top bar (zoom/cursor/dims), floating AI-detection
banner, multi-band gradient rows in the annotations sidebar, class-distribution
summary footer, and DOM-overlay bbox labels with affiliation icon + readiness
dot. Split VideoPlayer chrome out into the page-level controls row
(transport/frame-step/save/delete/AI-detect/mute/volume) and a new Scrubber
component; player events replace 200ms polling.

Other:
- Auth dev bypass via VITE_DEV_AUTH_BYPASS (gated on import.meta.env.DEV).
- Mount SavedAnnotationsProvider in App so AnnotationsPage doesn't crash.
- Extract hexToRgba to src/class-colors and time helpers to
  src/features/annotations/time.ts (dedup across CanvasEditor / Sidebar /
  AnnotationsPage).
- CanvasEditor: shallow-compare label chips before commit, NaN-guard
  annotation-time parser, cancel cursor RAF on unmount.
- AnnotationsPage: track AI-banner close timer, push initial volume to the
  <video> on media change, drop the duplicate parent muted state.
- Fixed sidebar widths (resize handles removed per design).

src/vite-env.d.ts

@@ -8,6 +8,8 @@ interface ImportMetaEnv {
   readonly VITE_OWM_API_KEY?: string
   readonly VITE_OWM_BASE_URL?: string
   readonly VITE_SATELLITE_TILE_URL?: string
+  /** Dev-only: when 'true', skip backend auth and inject a fake admin user. */
+  readonly VITE_DEV_AUTH_BYPASS?: string
 }
 
 interface ImportMeta {