refactor: compact pill-based filter toolbar

Merge the toggleable multi-line FilterBar and the separate ActiveFilterChips
strip into a single always-visible row of pills. Each filter category is a
pill that opens a small popover with its underlying control; when active, the
pill shows its current value inline (so the chips strip is redundant).

- New FilterPill primitive: outside-click + Escape to close, optional inline
  X to clear without opening the popover.
- FilterBar rebuilt out of pills for Date/Type/Rating/Color/Flag/Tags/Sort,
  with a Clear-all pill on the right when any filter is active.
- Drop filterBarOpen from filterStore, the SlidersHorizontal toggle from
  TopBar, the \\ shortcut from useKeyboardShortcuts, and the matching hint
  from KeyboardHints — the bar is always visible now.
- Delete ActiveFilterChips; its information lives inside the pills.

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

frontend/src/hooks/useKeyboardShortcuts.ts

@@ -1,7 +1,6 @@
 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, heaps as heapsApi, type Heap } from '../services/api'
 import { HEAPS_QUERY_KEY } from './useHeapsQuery'
 import { toast } from '../components/ToastContainer'
@@ -222,9 +221,7 @@ export function useKeyboardShortcuts(props: KeyboardShortcutsProps) {
   useHotkeys('tab', onToggleLeftSidebar, HK_OPTS)
   useHotkeys('i', onToggleRightSidebar, HK_OPTS)
 
-  // Filter bar toggle (\) and search focus (/ or Cmd/Ctrl+F).
-  useHotkeys('\\', () => useFilterStore.getState().toggleFilterBar(), HK_OPTS)
-
+  // Search focus (/ or Cmd/Ctrl+F).
   const focusSearch = () => {
     const el = document.getElementById('topbar-search') as HTMLInputElement | null
     el?.focus()