refactor: rename loupe to preview, bind to E and Space, fix empty viewer

The loupe view is now called "preview" everywhere — file paths, type
names, store actions, and the contextual hint pill. There's a single
preview action bound to E and Space (Enter is gone); double-click on a
thumbnail still works. Both shortcuts toggle: open from grid, close
from preview.

This commit also folds in the fix for the "preview shows nothing" bug
the user just hit:

- Extract usePhotosQuery into frontend/src/hooks/usePhotosQuery.ts so
  Timeline, PreviewView, and App.tsx all share one query — and one
  cache entry. Previously PreviewView and App.tsx looked the cache up
  by ['photos'], but the Timeline query key gained the filter params
  (['photos', filterParams]) when the filter bar shipped, so the
  lookup returned undefined and the preview rendered "No photo to
  display". App.tsx's getFirstPhotoId callback had the same bug.

- Harden PreviewImage: render the <img> immediately and overlay the
  spinner with absolute positioning, instead of toggling opacity-0 →
  opacity-100 on load. The previous opacity-toggle could leave the
  image stuck invisible if the load event raced with a key change.

- Add { preventDefault: true } to every useHotkeys call so single
  letter shortcuts (1-5, P, X, U) no longer leak into Firefox quick-
  find, and Cmd/Ctrl+F no longer triggers the browser find toolbar.

Files renamed:
  components/loupe/LoupeView.tsx       -> components/preview/PreviewView.tsx
  components/loupe/LoupeImage.tsx      -> components/preview/PreviewImage.tsx
  components/loupe/LoupeFilmstrip.tsx  -> components/preview/PreviewFilmstrip.tsx
  components/loupe/loupeSrc.ts         -> components/preview/previewSrc.ts

Symbol renames: openLoupe→openPreview, closeLoupe→closePreview, the
viewMode 'loupe' tag → 'preview', and all the LoupeXxx component and
helper exports.

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

frontend/src/components/preview/previewSrc.ts

@@ -0,0 +1,33 @@
+import { photos as photosApi } from '../../services/api'
+import type { Photo } from '../../types/photo'
+
+const VIDEO_EXTENSIONS = ['.mp4', '.mov', '.webm', '.mkv', '.m4v']
+
+export function isVideo(photo: Photo): boolean {
+  if (photo.media_type === 'video') return true
+  const lower = photo.filepath.toLowerCase()
+  return VIDEO_EXTENSIONS.some((ext) => lower.endsWith(ext))
+}
+
+/**
+ * Pick the best display URL for a still photo in the preview view.
+ *
+ * Always uses the /proxy endpoint, which the backend resolves to:
+ *   - the original file for web-safe formats (JPEG/PNG/WebP/GIF)
+ *   - a transcoded full-res WebP for RAW/HEIC/TIFF (cached on first hit)
+ *
+ * Videos go through `getVideoSrc` instead and use /original directly.
+ */
+export function getPreviewImageSrc(photo: Photo): string {
+  return photosApi.getProxyUrl(photo.id)
+}
+
+/** Fallback used when the proxy endpoint fails or 404s — shows the 1280px
+ *  large thumbnail so the user still sees something. */
+export function getPreviewFallbackSrc(photo: Photo): string {
+  return photosApi.getThumbnailUrl(photo.id, 'large')
+}
+
+export function getVideoSrc(photo: Photo): string {
+  return photosApi.getOriginalUrl(photo.id)
+}