v0.3.0: keyboard culling, toasts+undo, smart chips, selection gestures, guide

- keyboard-first culling in the library grid: cursor nav (arrows), 1-5 rating,
  P/X pick/reject labels, [ ] cycle color, Space preview, Delete trash
- toast + in-app confirm/prompt overlays replace all native alert/confirm/prompt;
  delete is now Gmail-style deferred trash with Undo
- smart quick-filter chips (today / this week / this year / best 4+ / videos),
  backed by a new mtime date-range filter in AssetQuery
- selection gestures: shift-click range, Ctrl/Cmd+A select-all, Esc clear
- mosaic: grid lines, lower default density/blend, click tile to view source
- user guide updated: library, culling, shortcuts, search/map/groups, mosaic

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/renderer/App.tsx

@@ -13,6 +13,7 @@ import { SearchView } from './components/SearchView'
 import { GroupsView } from './components/GroupsView'
 import { MapView } from './components/MapView'
 import { FileExplorer } from './components/FileExplorer'
+import { Overlays } from './overlays'
 import type { AppView } from './store'
 
 export default function App(): JSX.Element {
@@ -137,6 +138,9 @@ export default function App(): JSX.Element {
           <GroupsView />
         </main>
       )}
+
+      {/* 토스트 / 확인 모달 호스트 (전역) */}
+      <Overlays />
     </div>
   )
 }