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/store/photoStore.ts

@@ -1,7 +1,7 @@
 import { create } from 'zustand'
 import type { Photo } from '../types/photo'
 
-type ViewMode = 'grid' | 'loupe'
+type ViewMode = 'grid' | 'preview'
 
 interface PhotoStore {
   photos: Photo[]
@@ -19,8 +19,8 @@ interface PhotoStore {
   clearSelection: () => void
   setActivePhoto: (id: string | null) => void
   setViewMode: (mode: ViewMode) => void
-  openLoupe: (id: string) => void
-  closeLoupe: () => void
+  openPreview: (id: string) => void
+  closePreview: () => void
 }
 
 export const usePhotoStore = create<PhotoStore>((set) => ({
@@ -73,7 +73,7 @@ export const usePhotoStore = create<PhotoStore>((set) => ({
 
   setViewMode: (mode) => set({ viewMode: mode }),
 
-  openLoupe: (id) => set({ viewMode: 'loupe', activePhotoId: id }),
+  openPreview: (id) => set({ viewMode: 'preview', activePhotoId: id }),
 
-  closeLoupe: () => set({ viewMode: 'grid' }),
+  closePreview: () => set({ viewMode: 'grid' }),
 }))