add quick-open fuzzy file finder (Ctrl/Cmd+P)

A file picker overlay over the whole workspace, distinct from the
command palette. QuickOpenController holds the file list + a
subsequence fuzzy filter; the overlay loads the list via files.walk on
open, shows RecentFilesService entries on an empty query, and opens the
selection through a shared openWorkspaceFile helper (.md → markdown
reader bus, else editor.open) that the files panel now also routes
through, so recents stay in sync from every open site.

Bound to ctrl+p / meta+p with `when: !palette.open` so it never
collides with the palette's ctrl+p navigation; in-overlay arrows/enter/
escape reuse the palette's keymap-driven model via quickOpen.* intents.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/widgets/widgets.dart

@@ -28,6 +28,7 @@ export 'src/clide_surface.dart';
 export 'src/clide_tab_bar.dart';
 export 'src/multitab_controller.dart';
 export 'src/multitab_pane.dart';
+export 'src/quick_open_overlay.dart';
 export 'src/clide_tappable.dart';
 export 'src/clide_text.dart';
 export 'src/clide_tooltip.dart';