feat: filter bar with search, URL sync, and active-filter chips

Adds the spec §6.7 filter bar to the top of the timeline with:
  - Date range (native date inputs)
  - Media type chips (Photo / Video / RAW / HEIC, multi-select)
  - Min star rating (click to set, click again to clear)
  - Color label dots (red/orange/yellow/green/blue/purple, single)
  - Flag (any / picked / rejected / unflagged)
  - Clear-all button

Active filters surface as removable chips below the bar so they're
visible whether the bar is collapsed or open. The TopBar search input
is now wired to the same filter store with a 300ms debounce, and shows
a clear button when populated.

Filter state is the source of truth in a Zustand store and round-trips
through the URL via history.replaceState — bookmarkable and shareable
per spec §6.7. Hydrate happens once on mount; subsequent store changes
write back to ?q=&date_from=&… without navigation.

Timeline reads filter state, builds the backend params via
filtersToParams, and includes them in the React Query key so the cache
invalidates on every filter change. Also fixes a latent bug: Timeline
was sending limit=1000&offset=0, which the backend silently ignores —
swapped to page=1&per_page=500 with explicit sort=taken_at&order=desc.

New keyboard shortcuts:
  - \\  toggles the filter bar
  - /   focuses the TopBar search input
  - Cmd/Ctrl+F  same as /

Filter button in the TopBar now lights up when filters are active or
the bar is open.

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

frontend/src/hooks/useKeyboardShortcuts.ts

@@ -1,6 +1,7 @@
 import { useHotkeys } from 'react-hotkeys-hook'
 import { useMutation, useQueryClient } from '@tanstack/react-query'
 import { usePhotoStore } from '../store/photoStore'
+import { useFilterStore } from '../store/filterStore'
 import { photos as photosApi } from '../services/api'
 
 interface KeyboardShortcutsProps {
@@ -66,6 +67,21 @@ export function useKeyboardShortcuts(props: KeyboardShortcutsProps) {
     onToggleRightSidebar()
   })
 
+  // Filter bar toggle (\) and search focus (/ or Cmd/Ctrl+F).
+  useHotkeys('\\', (e) => {
+    e.preventDefault()
+    useFilterStore.getState().toggleFilterBar()
+  })
+
+  const focusSearch = (e: KeyboardEvent) => {
+    e.preventDefault()
+    const el = document.getElementById('topbar-search') as HTMLInputElement | null
+    el?.focus()
+    el?.select()
+  }
+  useHotkeys('/', focusSearch)
+  useHotkeys('mod+f', focusSearch)
+
   // G always returns to grid (closes loupe if open).
   useHotkeys('g', () => {
     closeLoupe()