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/surfaces/DOCUMENTATION.md

@@ -20,6 +20,13 @@ edge (`components/layout/ContextPanelRail.tsx`) and rendered by
 - Rail order is user-reorderable and persisted globally in
   `useUIStore.contextRailOrder`; `sortContextSurfaces` applies it on top of the
   registry's default order and appends any missing surfaces.
+- `getVisibleContextRailSurfaces` is the single visibility filter shared by the
+  rail and the global surface-switch shortcut (`switch_context_surface` in
+  `lib/shortcuts.ts`): it drops the plan surface unless plan mode is enabled,
+  drops the walkthrough on VS Code and below `WALKTHROUGH_MIN_WIDTH`, and hides
+  `has-content` surfaces until a tab of their mode exists. Both consumers use
+  it so the digit shown on a rail badge always maps to the same surface the
+  shortcut opens.
 
 ## Adding a surface