feat(workspace): one global switcher anchor in the shell sidebar

The switcher used to be embedded per page (Flows/Runs/Audit/
Approvals) — invisible on the other five pages and sitting in a
different corner depending on the page (persona review 2026-08-27,
consensus finding). It now lives ONCE in the sidebar, above the
destinations: active project/area always visible, opens the same
menu everywhere, Cmd+P from anywhere. The shell listens to the
workspace, so the sidebar endpoint label can no longer lag a
sealed switch until the next health tick.

Also in this rebuild:

* Stopped sealed areas ask before starting ("Start area X?") —
  a context switch must never boot a hub daemon as a click
  side-effect; running areas keep switching with one click.
* The switcher tooltip told a wrong scope ("filters this view") —
  it now says the choice applies everywhere and stamps new runs.
* The aggregated sealed row explains itself in place (names can
  reveal client identities) and links to the Settings toggle
  (Settings dialog gained an initialCategory jump).
* The active entry carries a checkmark in the menu.
* The Cmd+K palette knows projects and areas, ranked by recent
  use; sealed names honour the privacy setting — while hidden,
  the palette offers the guarded picker instead of the names.
* The runs empty state names the active project filter as the
  cause ("No runs in project X" + show-all action) instead of
  claiming the feature is off.

Tests updated to the anchor and made hermetic (scriptable
projects on the fake hub, sealed-area fake); new coverage for the
checkmark, the why-line, and the start confirmation.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

test/support/fake_hub.dart

@@ -183,9 +183,14 @@ class FakeHubService extends Fake implements HubService {
   @override
   Future<List<SavedFlow>> listFlows() => _async('listFlows', () => const []);
 
+  /// Registry projects [listProjects] answers with — scriptable so
+  /// workspace suites can keep their seed across the switcher's
+  /// open-menu refresh.
+  List<ProjectRef> projects = const [];
+
   @override
   Future<List<ProjectRef>> listProjects() =>
-      _async('listProjects', () => const []);
+      _async('listProjects', () => projects);
 
   @override
   Stream<AuditEvent> streamEvents({