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/styles/index.css

@@ -34,6 +34,18 @@ html.dark body {
   font-family: 'Cascadia Code', 'Consolas', monospace;
 }
 
+/* 토스트 등장 */
+@keyframes fadeIn {
+  from {
+    opacity: 0;
+    transform: translateY(6px);
+  }
+  to {
+    opacity: 1;
+    transform: translateY(0);
+  }
+}
+
 /* darktable식 얇은 스크롤바 */
 ::-webkit-scrollbar {
   width: 10px;