feat: loupe view with zoom, pan, video, and filmstrip

Adds the Lightroom-style full-screen single-photo viewer (spec §6.11).
Open with E / Enter / double-click; navigate with arrow keys; Esc returns
to grid. Filmstrip at the bottom auto-scrolls the active cell into view.

Display:
- Stills source from /photos/{id}/proxy so RAW/HEIC are decoded server-
  side; large thumbnail is the onError fallback only.
- Videos render in a <video controls> sourced from /original.
- Continuous wheel zoom (1×–8×, ~15% per tick) with click-drag pan when
  zoomed past fit. Z toggles between fit and natural-resolution
  (computed from naturalWidth / clientWidth); a second Z snaps back.
- Live percentage indicator in the bottom-center while zoomed.

Polish:
- Neighbor preloading via new Image() when currentIndex changes so arrow
  nav feels instant (skips videos).
- Focus trap with role=dialog, aria-modal, focus-on-mount, restore-on-
  unmount, and Tab cycling among focusable children.

Plumbing:
- New canonical Photo TS interface in types/photo.ts; removes the three
  duplicated definitions in PhotoThumbnail/Timeline/photoStore.
- photoStore gains viewMode + openLoupe/closeLoupe.
- useKeyboardShortcuts wires E/Enter/G to open/close, gates rating and
  flag stubs with { enabled: viewMode === 'grid' } so they're inert in
  loupe.
- App.tsx mounts <LoupeView/> as a z-40 overlay covering TopBar, gates
  the auto-open right sidebar effect on viewMode === 'grid' so leaving
  loupe doesn't fight the user's prior sidebar state.
- PhotoThumbnail gains an onDoubleClick prop wired to openLoupe.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/store/photoStore.ts

@@ -1,20 +1,7 @@
 import { create } from 'zustand'
+import type { Photo } from '../types/photo'
 
-interface Photo {
-  id: string
-  filename: string
-  filepath: string
-  media_type: string
-  width?: number
-  height?: number
-  taken_at?: string
-  thumb_small?: string
-  thumb_medium?: string
-  thumb_large?: string
-  rating: number
-  is_picked: boolean
-  is_rejected: boolean
-}
+type ViewMode = 'grid' | 'loupe'
 
 interface PhotoStore {
   photos: Photo[]
@@ -22,7 +9,8 @@ interface PhotoStore {
   activePhotoId: string | null
   lastSelectedIndex: number | null
   rangeStartIndex: number | null
-  
+  viewMode: ViewMode
+
   setPhotos: (photos: Photo[]) => void
   selectPhoto: (id: string, index: number) => void
   togglePhotoSelection: (id: string, index: number) => void
@@ -30,6 +18,9 @@ interface PhotoStore {
   deselectPhoto: (id: string) => void
   clearSelection: () => void
   setActivePhoto: (id: string | null) => void
+  setViewMode: (mode: ViewMode) => void
+  openLoupe: (id: string) => void
+  closeLoupe: () => void
 }
 
 export const usePhotoStore = create<PhotoStore>((set) => ({
@@ -38,7 +29,8 @@ export const usePhotoStore = create<PhotoStore>((set) => ({
   activePhotoId: null,
   lastSelectedIndex: null,
   rangeStartIndex: null,
-  
+  viewMode: 'grid',
+
   setPhotos: (photos) => set({ photos }),
   
   selectPhoto: (id, index) => set({
@@ -78,4 +70,10 @@ export const usePhotoStore = create<PhotoStore>((set) => ({
   }),
   
   setActivePhoto: (id) => set({ activePhotoId: id }),
+
+  setViewMode: (mode) => set({ viewMode: mode }),
+
+  openLoupe: (id) => set({ viewMode: 'loupe', activePhotoId: id }),
+
+  closeLoupe: () => set({ viewMode: 'grid' }),
 }))