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/components/timeline/PhotoThumbnail.tsx

@@ -2,33 +2,21 @@ import { useState, useEffect, useCallback, useRef } from 'react'
 import { Star, Check, X, RefreshCw } from 'lucide-react'
 import clsx from 'clsx'
 import { photos as photosApi } from '../../services/api'
+import type { Photo } from '../../types/photo'
 
 // Auto-retry schedule (ms). Backend generates thumbs on-demand via Celery, so
 // first hit often 404s. Try a few times with backoff before giving up.
 const AUTO_RETRY_DELAYS = [1500, 3500, 6000]
 
-interface Photo {
-  id: string
-  filepath: string
-  filename: string
-  width: number | null
-  height: number | null
-  taken_at: string | null
-  rating: number
-  is_picked: boolean
-  is_rejected: boolean
-  file_hash: string
-  media_type: string
-}
-
 interface PhotoThumbnailProps {
   photo: Photo
   size: number
   isSelected: boolean
   onClick: (e: React.MouseEvent) => void
+  onDoubleClick?: (e: React.MouseEvent) => void
 }
 
-export function PhotoThumbnail({ photo, size, isSelected, onClick }: PhotoThumbnailProps) {
+export function PhotoThumbnail({ photo, size, isSelected, onClick, onDoubleClick }: PhotoThumbnailProps) {
   const [imageError, setImageError] = useState(false)
   const [imageLoaded, setImageLoaded] = useState(false)
   const [retryCount, setRetryCount] = useState(0)
@@ -108,7 +96,8 @@ export function PhotoThumbnail({ photo, size, isSelected, onClick }: PhotoThumbn
         height: displayHeight,
       }}
       onClick={onClick}
-      title="Click to select • Shift+Click for range • Ctrl+Click to add"
+      onDoubleClick={onDoubleClick}
+      title="Click to select • Double-click to open • Shift+Click for range • Ctrl+Click to add"
     >
       {/* Thumbnail Image */}
       {!imageError ? (