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/data/system_actions.dart

@@ -205,6 +205,17 @@ class SystemActions {
     return _runFai(args);
   }
 
+  /// Start a sealed project's isolated hub instance
+  /// (`chain project start <slug>`). Detached daemon on the
+  /// instance's fixed port — returns once the CLI has spawned it.
+  /// Used by Studio's workspace switch when the operator selects a
+  /// stopped sealed area.
+  static Future<({bool ok, String stdout, String stderr})> chainProjectStart(
+    String slug,
+  ) async {
+    return _runFai(['project', 'start', slug]);
+  }
+
   static Future<({bool ok, String stdout, String stderr})> _runFai(
     List<String> args,
   ) async {