Add command palette and command registry

- Commands (with optional shortcuts and availability checks) are registered in one place;
  global shortcuts are dispatched from it, and plugins can use registerCommand later
- The quick switcher lists commands when the input starts with '>' (VS Code style),
  opened with Ctrl+Shift+P or F1 (Firefox reserves Ctrl+Shift+P), plus buttons in the
  sidebar footer and the mobile top bar; recently used commands come first
- Built-in commands: quick open, search, new note/folder, save, rename, delete, close tab,
  trash, log out
- Mount the trash dialog at the workspace level so it is not positioned inside the
  transformed mobile drawer

web/src/app/app.css

@@ -217,6 +217,7 @@ button {
 .rm-switcher-item.active { background: var(--rm-active); }
 .rm-switcher-title { color: var(--rm-text); white-space: nowrap; overflow: hidden; text-overflow: ellipsis; }
 .rm-switcher-dir { flex: 1; min-width: 0; font-size: 12px; white-space: nowrap; overflow: hidden; text-overflow: ellipsis; text-align: right; }
+.rm-switcher-keys span { padding: 1px 6px; border: 1px solid var(--rm-divider); border-radius: 4px; font-family: inherit; }
 .rm-sidebar-foot { display: flex; gap: 4px; padding: 8px; border-top: 1px solid var(--rm-divider); }
 .rm-foot-btn {
   flex: 1;