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/main.dart

@@ -12,6 +12,7 @@ import 'package:flutter/services.dart';
 import 'data/chain_log.dart';
 import 'data/error_presentation.dart';
 import 'data/hub.dart';
+import 'data/workspace.dart';
 import 'data/system_actions.dart';
 import 'data/theme_plugin.dart';
 import 'l10n/app_localizations.dart';
@@ -425,7 +426,11 @@ class StudioShellState extends State<StudioShell> {
         }
       } catch (_) {/* best-effort */}
       try {
-        final pending = await HubService.instance.pendingApprovals();
+        // The badge counts the active workspace's pending approvals
+        // (empty slug = all projects), matching the Approvals page.
+        final pending = await HubService.instance.pendingApprovals(
+          project: Workspace.instance.activeSlug,
+        );
         if (!mounted) return;
         if (_pendingApprovals != pending.length) {
           setState(() => _pendingApprovals = pending.length);