feat(ui): numbered context-panel surface switching with configurable prefix

- Add switch_context_surface shortcut (default Cmd/Ctrl + 1..9, 0 for the 10th
  surface) that opens/closes/switches context panel rail surfaces by their
  visible order, configurable and persisted in Settings -> Shortcuts.
- Show order-number badges on rail icons while the modifier is held >500ms;
  dismiss on release, blur, or a number press until the next press-and-hold.
- Remove the legacy mod+2/3/4 (diff/terminal/git) and switch_tab_1..9 bindings
  so numbered surface switching goes only through the new mechanism.
- Replace the help-dialog 'Switch Project' row with the surface-switch row and
  update the shortcuts footer/header icons to the command icon.

packages/ui/src/lib/i18n/messages/ja.ts

@@ -1707,7 +1707,7 @@ export const dict: Record<I18nKey, string> = {
   'helpDialog.item.toggleTerminalExpanded': 'ターミナル展開の切り替え',
   'helpDialog.item.togglePlanContextPanel': '計画コンテキストパネルの切り替え',
   'helpDialog.item.cycleTheme': 'テーマ切り替え（ライト→ダーク→システム）',
-  'helpDialog.item.switchProject': 'プロジェクトを切り替え',
+  'helpDialog.item.switchContextSurface': 'コンテキストパネルのサーフェスを切り替え（数字キー）',
   'helpDialog.item.toggleServicesMenu': 'サービスの切り替え',
   'helpDialog.item.cycleServicesTab': 'サービス変数の切り替え',
   'helpDialog.item.openSettings': '設定を開く',