feat(settings): settings panel shell + settings.open command (T-445)

Lay the foundation for the schema-driven settings UI (epic T-444). A new
`settings.open` command (⌘`,`, plus a File-menu and command-palette entry)
opens a centered Settings modal over the dimmed app via the dialog router,
built from the modalSurface* tokens (D-7, no Material). The shell frames the
two regions later tickets fill in — the category rail (T-447) and the
scrolling carded panel (T-448) — and dismisses on ✕, Esc, or barrier tap.
With no category registered yet it shows its empty state, which is the
correct runtime state.

Flesh out the `builtin.settings-ui` stub (was 0.0.0-stub) into a real
extension; ship its en-US i18n catalog. Relabel the theme picker's
`theme.pick` command title from "Settings…" to "Theme…" so the two no
longer collide in the palette (the picker folds into the new panel's
Appearance category in T-452).

Tests: command + ⌘`,` binding registered, shell renders, Esc and close
both dismiss.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/builtin/menubar/src/extension.dart

@@ -75,6 +75,8 @@ List<TopMenu> buildClideMenuTree() => [
       const MenuCommandItem('file.openFolder', fallbackTitle: 'Open Folder…'),
       const MenuCommandItem('file.newWindow', fallbackTitle: 'New Window'),
       const MenuSeparator(),
+      const MenuCommandItem('settings.open', fallbackTitle: 'Settings…'),
+      const MenuSeparator(),
       MenuCommandItem('file.closeWorkspace', fallbackTitle: 'Close Project', enabledWhen: (s) => s.project.isOpen),
     ],
   ),