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/de.settings.ts

@@ -1043,6 +1043,8 @@ export const settingsDict = {
   'settings.openchamber.keyboardShortcuts.action.toggle_right_sidebar.label': 'Rechte Seitenleiste umschalten',
   'settings.openchamber.keyboardShortcuts.action.open_right_sidebar_git.label': 'Git-Tab der rechten Seitenleiste öffnen',
   'settings.openchamber.keyboardShortcuts.action.open_right_sidebar_files.label': 'Datei-Tab der rechten Seitenleiste öffnen',
+  'settings.openchamber.keyboardShortcuts.action.switch_context_surface.label': 'Kontextpanel-Oberfläche wechseln',
+  'settings.openchamber.keyboardShortcuts.action.switch_context_surface.suffix': ' + 1…0',
   'settings.openchamber.keyboardShortcuts.action.new_chat.label': 'Neue Sitzung',
   'settings.openchamber.keyboardShortcuts.action.new_chat_worktree.label': 'Neuer Worktree-Entwurf',
   'settings.openchamber.keyboardShortcuts.action.new_mini_chat.label': 'Neues Mini-Chat-Fenster',