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/components/session/sidebar/SidebarFooter.tsx

@@ -44,7 +44,7 @@ export function SidebarFooter({
           <Tooltip>
             <TooltipTrigger asChild>
               <button type="button" onClick={onOpenShortcuts} className={footerButtonClassName} aria-label={t('sessions.sidebar.footer.actions.shortcuts')}>
-                <Icon name="question" className="h-4.5 w-4.5" />
+                <Icon name="command" className="h-4.5 w-4.5" />
               </button>
             </TooltipTrigger>
             <TooltipContent side="top" sideOffset={4}><p>{t('sessions.sidebar.footer.actions.shortcuts')}</p></TooltipContent>