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/theme_picker/src/extension.dart

@@ -20,10 +20,12 @@ class ThemePickerExtension extends ClideExtension {
 
   @override
   List<ContributionPoint> get contributions => [
-    // Opens the settings modal (T-238). Command id kept as `theme.pick`
-    // (the welcome theme-link and other callers reference it); ⌘K opens
-    // Settings, whose only section today is the theme picker.
-    CommandContribution(id: 'theme.pick', command: 'theme.pick', title: 'Settings…', defaultBinding: 'ctrl+k', run: _pick),
+    // Opens the theme picker modal (T-238). Command id kept as `theme.pick`
+    // (the welcome theme-link and other callers reference it). Titled
+    // "Theme…" to disambiguate from the schema-driven Settings panel's
+    // `settings.open` (T-444); the theme picker folds into that panel's
+    // Appearance category in T-452.
+    CommandContribution(id: 'theme.pick', command: 'theme.pick', title: 'Theme…', defaultBinding: 'ctrl+k', run: _pick),
     // Always-visible switcher in the far-right status bar (T-234).
     // priority >= 100 places it in the right group; registered after
     // ipc-status so it sits to its right.