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/l10n/app_en.arb

@@ -1723,5 +1723,16 @@
   "runsPhaseFailed": "Failed",
   "runsPhaseCancelled": "Cancelled",
   "runsPhaseUnknown": "Unknown",
-  "navRuns": "Runs"
+  "navRuns": "Runs",
+  "workspaceSealedHeader": "SEALED AREAS",
+  "workspaceSealedRunning": "running",
+  "workspaceSealedStopped": "stopped",
+  "workspaceSealedStarting": "Starting sealed area “{name}”…",
+  "@workspaceSealedStarting": {"placeholders": {"name": {"type": "String"}}},
+  "workspaceSealedStarted": "Started sealed area “{name}”.",
+  "@workspaceSealedStarted": {"placeholders": {"name": {"type": "String"}}},
+  "workspaceSealedSwitchFailed": "Could not switch to the sealed area: {error}",
+  "@workspaceSealedSwitchFailed": {"placeholders": {"error": {"type": "String"}}},
+  "sealedIdentityBar": "Sealed area — isolated hub, own data and audit chain",
+  "sealedLeave": "Leave"
 }