feat: workspace switcher, per-project filters and run stamping (stage 1)

Multi-project stage 1 against the shared hub (platform design
docs/architecture/projects.md, § Studio):

- ChainWorkspaceSwitcher in the Audit + Approvals AppBars: lists the
  registry (colour dot per project, shield for protected, honesty
  tooltip), 'All projects' stays reachable — a filter, not a jail.
  Selection is persisted and shared via the Workspace notifier.
- Audit page: list query AND live stream re-scoped hub-side on switch.
- Approvals page: pending + history scoped; the sidebar badge counts
  the active workspace's pending approvals.
- Flow runs are stamped with the active workspace; a flow file
  carrying its own project: keeps it (file wins, CLI semantics).
- Data layer: listProjects/ProjectRef; project fields on AuditEvent,
  PendingApproval(+Record), SavedFlow; project params through
  HubService. l10n DE+EN. Widget tests for the switcher contract.

Visual verification (light+dark screenshots) still pending — the
shared desktop was in active use; code paths are covered by
flutter test (26 green).

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

lib/widgets/widgets.dart

@@ -19,3 +19,4 @@ export 'chain_settings_dialog.dart';
 export 'chain_stores_dialog.dart';
 export 'chain_status_dot.dart';
 export 'chain_system_ai_editor.dart';
+export 'chain_workspace_switcher.dart';