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>

CHANGELOG.md

@@ -16,6 +16,20 @@ heading, and (b) bumping `pubspec.yaml` `version:` in the same commit.
 
 ## [Unreleased]
 
+### Added
+
+- **Settings panel shell.** A new `settings.open` command (⌘`,`, plus a File
+  menu and command-palette entry) opens a centered Settings modal over the
+  dimmed app — the foundation of the schema-driven settings UI. It frames the
+  category rail and scrolling carded panel that later work fills in; closes on
+  ✕, Esc, or a barrier tap. (T-445)
+
+### Changed
+
+- **Theme picker relabelled "Theme…".** The ⌘K theme picker's command title
+  changed from "Settings…" to "Theme…" so it no longer collides with the new
+  Settings panel in the palette; behaviour is unchanged. (T-445)
+
 ## [2.6.0] — 2026-06-16
 
 ### Added