fix(ui): gate Mission Control submit-for-review, add retry, fix event leak

Submit for Review is now disabled while a ConPTY pane is starting, has
failed to launch, or has already exited, and MissionControlViewModel
guards against a rapid double-click racing two SubmitTaskForReviewAsync
calls. A failed launch no longer permanently occupies its TaskId dedupe
slot -- a Retry button re-fetches the launch spec and restarts the pane
in place. CloseConPtySession/Dispose now also unsubscribe
SubmitForReviewRequested, matching the other pane event handlers. Also
fixes the pre-existing nullable-dereference warning in
IslandsShellViewModel.SyncInteractiveSessionChips.

src/ClaudeDo.Localization/locales/de.json

@@ -285,6 +285,8 @@
     "submitForReviewFailed": "Einreichen zum Review fehlgeschlagen: {0}",
     "submitForReview": "Zum Review einreichen",
     "submitForReviewTip": "Diesen Worktree committen und den Task ins Review bringen, damit er gemergt werden kann",
+    "retry": "Erneut versuchen",
+    "retryTip": "Diese Sitzung erneut starten",
     "planningTitleSuffix": " (Planung)",
     "question": {
       "title": "Claude fragt nach",