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/kernel/src/i18n/catalog/builtin.settings-ui_en_us.json

@@ -0,0 +1,7 @@
+{
+  "modal.title": { "translation": "Settings" },
+  "modal.close": { "translation": "Close" },
+  "modal.close.hint": { "translation": "Close settings without changing anything" },
+  "rail.header": { "translation": "Categories" },
+  "panel.empty": { "translation": "No settings categories are registered yet." }
+}