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/chain_workspace_switcher.dart

@@ -0,0 +1,163 @@
+// ChainWorkspaceSwitcher — the AppBar workspace (project) control.
+//
+// One control, two mechanics (docs/architecture/projects.md,
+// § Studio): for open/protected projects the choice sets the page
+// filter AND the label new runs are stamped with. "All projects"
+// stays reachable — a filter, not a jail. Sealed areas never
+// appear here (own hub instance; stage-3 connection switch).
+//
+// The project colour is a marking dot only — Studio's blue stays
+// the app accent (registered design deviation).
+
+import 'package:flutter/material.dart';
+
+import '../data/hub.dart';
+import '../data/workspace.dart';
+import '../l10n/app_localizations.dart';
+import '../theme/tokens.dart';
+
+class ChainWorkspaceSwitcher extends StatelessWidget {
+  const ChainWorkspaceSwitcher({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return ListenableBuilder(
+      listenable: Workspace.instance,
+      builder: (context, _) {
+        final l = AppLocalizations.of(context)!;
+        final theme = Theme.of(context);
+        final ws = Workspace.instance;
+        final active = ws.active;
+        final label = ws.isAll
+            ? l.workspaceAll
+            : (active?.name ?? ws.activeSlug);
+
+        return Tooltip(
+          message: l.workspaceSwitcherTooltip,
+          child: PopupMenuButton<String>(
+            onOpened: ws.refresh,
+            onSelected: ws.setActive,
+            itemBuilder: (context) => [
+              PopupMenuItem(
+                value: '',
+                child: Row(
+                  children: [
+                    Icon(
+                      Icons.grid_view_outlined,
+                      size: 16,
+                      color: theme.colorScheme.onSurfaceVariant,
+                    ),
+                    const SizedBox(width: ChainSpace.sm),
+                    Text(l.workspaceAll),
+                  ],
+                ),
+              ),
+              if (ws.projects.isNotEmpty) const PopupMenuDivider(),
+              for (final p in ws.projects)
+                PopupMenuItem(
+                  value: p.slug,
+                  child: Row(
+                    children: [
+                      _ProjectDot(project: p),
+                      const SizedBox(width: ChainSpace.sm),
+                      Flexible(
+                        child: Text(p.name, overflow: TextOverflow.ellipsis),
+                      ),
+                      if (p.isProtected) ...[
+                        const SizedBox(width: ChainSpace.sm),
+                        Tooltip(
+                          message: l.workspaceProtectedHint,
+                          child: Icon(
+                            Icons.shield_outlined,
+                            size: 14,
+                            color: theme.colorScheme.onSurfaceVariant,
+                          ),
+                        ),
+                      ],
+                    ],
+                  ),
+                ),
+            ],
+            child: Container(
+              padding: const EdgeInsets.symmetric(
+                horizontal: ChainSpace.md,
+                vertical: 5,
+              ),
+              decoration: BoxDecoration(
+                color: theme.colorScheme.surfaceContainerHighest,
+                borderRadius: BorderRadius.circular(ChainRadius.sm),
+              ),
+              child: Row(
+                mainAxisSize: MainAxisSize.min,
+                children: [
+                  if (active != null) ...[
+                    _ProjectDot(project: active),
+                    const SizedBox(width: 6),
+                  ] else ...[
+                    Icon(
+                      Icons.grid_view_outlined,
+                      size: 14,
+                      color: theme.colorScheme.onSurfaceVariant,
+                    ),
+                    const SizedBox(width: 6),
+                  ],
+                  ConstrainedBox(
+                    constraints: const BoxConstraints(maxWidth: 160),
+                    child: Text(
+                      label,
+                      overflow: TextOverflow.ellipsis,
+                      style: theme.textTheme.labelMedium,
+                    ),
+                  ),
+                  if (active?.isProtected ?? false) ...[
+                    const SizedBox(width: 4),
+                    Icon(
+                      Icons.shield_outlined,
+                      size: 13,
+                      color: theme.colorScheme.onSurfaceVariant,
+                    ),
+                  ],
+                  const SizedBox(width: 2),
+                  Icon(
+                    Icons.arrow_drop_down,
+                    size: 18,
+                    color: theme.colorScheme.onSurfaceVariant,
+                  ),
+                ],
+              ),
+            ),
+          ),
+        );
+      },
+    );
+  }
+}
+
+/// The project's registry colour as a small marking dot. Falls
+/// back to the theme's outline colour when the project has none.
+class _ProjectDot extends StatelessWidget {
+  final ProjectRef project;
+
+  const _ProjectDot({required this.project});
+
+  @override
+  Widget build(BuildContext context) {
+    final theme = Theme.of(context);
+    return Container(
+      width: 10,
+      height: 10,
+      decoration: BoxDecoration(
+        color: _parseHex(project.color) ?? theme.colorScheme.outline,
+        shape: BoxShape.circle,
+      ),
+    );
+  }
+
+  static Color? _parseHex(String hex) {
+    final h = hex.replaceFirst('#', '');
+    if (h.length != 6) return null;
+    final v = int.tryParse(h, radix: 16);
+    if (v == null) return null;
+    return Color(0xFF000000 | v);
+  }
+}