implement collapse spine, focus mode, keyboard shortcuts, context rail

Collapsed side panels render as a 12px vertical spine with rotated
label, hover highlight, and badge dot (D-051). Focus mode snapshots
and restores the full layout arrangement (D-052). Fourteen new
commands in the default-layout extension: collapse toggles
(Ctrl+Shift+1/3), panel focus (Ctrl+1/2/3), focus mode (Ctrl+.),
sidebar section switching (Alt+1-5), Escape dismiss (D-054). Right
panel gets a bottom icon rail matching the sidebar pattern (D-047).

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

app/lib/widgets/widgets.dart

@@ -13,6 +13,7 @@ export 'src/clide_palette.dart';
 export 'src/clide_pane_chrome.dart';
 export 'src/clide_pty_view.dart';
 export 'src/clide_scrollbar.dart';
+export 'src/clide_spine.dart';
 export 'src/clide_surface.dart';
 export 'src/clide_tab_bar.dart';
 export 'src/clide_text.dart';