feat: sealed-area connection switch + identity bar (multi-project stage 3)

The workspace switcher now lists the operator's sealed areas (read from
~/.chain/sealed/ manifests, the same source the CLI uses) below the
shared projects, each with a lock icon and a running/stopped status.
Selecting one is a real connection switch: Studio reconnects its hub
client to the area's own port with a full state reload — one window,
one truth. A stopped area is started first (chain project start) with a
visible notice; a failure surfaces as a copyable error and rolls back to
the shared hub.

While in a sealed area an identity bar under the AppBar is painted in
the area's accent colour and names it, with a one-click Leave back to
the shared hub. The area colour is marking, not theming — Studio's blue
stays the app accent. Selecting a shared project from inside an area
switches the connection back first. The sealed connection is never
persisted across restarts.

New: SealedAreaService (manifest + PID discovery), Workspace sealed
switch logic, ChainSealedIdentityBar, SystemActions.chainProjectStart.
l10n DE+EN. flutter analyze clean; 33 tests green (switcher lists sealed
with lock+status, pill shows active area, identity bar renders in the
area colour). Runtime plumbing (discovery, start, endpoint, reach)
verified headlessly against real sealed instances under a redirected
HOME; the identity-bar screenshot is deferred (display click-automation
failed after sleep on the shared desktop — an environment issue, not a
code gap; the visible components are widget-tested).

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

lib/widgets/chain_sealed_identity_bar.dart

@@ -0,0 +1,75 @@
+// ChainSealedIdentityBar — the identity strip shown under the AppBar
+// while Studio is connected to a sealed area.
+//
+// Painted in the area's accent colour so "which area am I in?" is
+// answerable at a glance (one window, one truth). The area colour is
+// marking, not theming — Studio's blue stays the app accent
+// everywhere else. Offers a one-click way back to the shared hub.
+// Renders nothing when Studio is on the shared hub.
+
+import 'package:flutter/material.dart';
+
+import '../data/workspace.dart';
+import '../l10n/app_localizations.dart';
+import '../theme/tokens.dart';
+import 'chain_workspace_switcher.dart' show parseAreaColor;
+
+class ChainSealedIdentityBar extends StatelessWidget {
+  const ChainSealedIdentityBar({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return ListenableBuilder(
+      listenable: Workspace.instance,
+      builder: (context, _) {
+        final area = Workspace.instance.activeSealed;
+        if (area == null) return const SizedBox.shrink();
+        final theme = Theme.of(context);
+        final l = AppLocalizations.of(context)!;
+        final accent = parseAreaColor(area.color) ?? theme.colorScheme.primary;
+        // A readable foreground on the tinted bar in both themes.
+        final onAccent =
+            accent.computeLuminance() > 0.5 ? Colors.black87 : Colors.white;
+        return Material(
+          color: accent,
+          child: Padding(
+            padding: const EdgeInsets.symmetric(
+              horizontal: ChainSpace.lg,
+              vertical: 6,
+            ),
+            child: Row(
+              children: [
+                Icon(Icons.lock_outline, size: 15, color: onAccent),
+                const SizedBox(width: ChainSpace.sm),
+                Text(
+                  area.name,
+                  style: theme.textTheme.labelLarge?.copyWith(
+                    color: onAccent,
+                    fontWeight: FontWeight.w600,
+                  ),
+                ),
+                const SizedBox(width: ChainSpace.md),
+                Expanded(
+                  child: Text(
+                    l.sealedIdentityBar,
+                    style: theme.textTheme.bodySmall?.copyWith(
+                      color: onAccent.withValues(alpha: 0.85),
+                    ),
+                    overflow: TextOverflow.ellipsis,
+                  ),
+                ),
+                const SizedBox(width: ChainSpace.sm),
+                TextButton.icon(
+                  onPressed: () => Workspace.instance.switchToShared(),
+                  style: TextButton.styleFrom(foregroundColor: onAccent),
+                  icon: const Icon(Icons.logout, size: 15),
+                  label: Text(l.sealedLeave),
+                ),
+              ],
+            ),
+          ),
+        );
+      },
+    );
+  }
+}