fix(ui): surface worker connect/restart failures instead of no-opting

WorkerConnectionModalViewModel's StartWorker/RerunInstaller swallowed
missing-exe and Process.Start failures behind an empty catch and closed
the dialog anyway, looking like success. Now both show an inline error
and keep the dialog open; only a real launch closes it.

IslandsShellViewModel.RestartWorkerAsync never checked whether the
relaunched worker actually stayed up, and its status property was never
bound in any view. RestartWorkerService now fails if the new process
exits within a short survival window, and the outcome routes through
the footer error/success strip instead of the dead RestartWorkerStatus.

InstallArtifactLocator.Find() is now virtual and its two subclasses
unsealed so tests can fake exe resolution without touching the real
filesystem/registry.

src/ClaudeDo.Localization/locales/de.json

@@ -407,7 +407,10 @@
       "body": "ClaudeDo kann den Hintergrund-Worker nicht erreichen. Normalerweise wird er bei der Anmeldung automatisch gestartet. Du kannst ihn jetzt starten oder neu installieren, falls das Problem bestehen bleibt.",
       "dismiss": "Ausblenden",
       "rerunInstaller": "Installer erneut ausführen",
-      "startWorker": "Worker starten"
+      "startWorker": "Worker starten",
+      "workerNotFound": "Worker-Programm nicht gefunden. Installation prüfen.",
+      "installerNotFound": "Installer nicht gefunden. Installation prüfen.",
+      "startFailed": "Start fehlgeschlagen: {0}"
     },
     "listSettings": {
       "title": "LISTENEINSTELLUNGEN",
@@ -777,7 +780,7 @@
   },
   "vm": {
     "connection": { "online": "Online", "connecting": "Verbinden…", "offline": "Offline" },
-    "shell": { "restartingWorker": "Worker wird neu gestartet…", "unexpectedError": "Unerwarteter Fehler: {0}" },
+    "shell": { "unexpectedError": "Unerwarteter Fehler: {0}" },
     "mainWindow": { "closeConfirm": { "message": "{0} Mission-Control-Sitzung(en) laufen noch. Beim Beenden werden sie abgebrochen. Trotzdem schließen?" } },
     "agentStatus": { "idle": "Leerlauf", "queued": "In Warteschlange", "running": "Läuft", "review": "Prüfung", "children": "Wartet auf Teilaufgaben", "done": "Fertig", "failed": "Fehlgeschlagen", "cancelled": "Abgebrochen" },
     "taskStatus": { "idle": "Leerlauf", "queued": "In Warteschlange", "running": "Läuft", "waitingForReview": "Wartet auf Prüfung", "waitingForChildren": "Wartet auf Teilaufgaben", "done": "Fertig", "failed": "Fehlgeschlagen", "cancelled": "Abgebrochen", "parked": "Geparkt", "interactive": "Interaktiv", "waiting": "Wartet" },