fix(ui): show a banner when a ConPTY session dies after startup

InteractiveTerminalViewModel.HasExitedAfterStartup now covers every
post-startup death that ExitedAfterStartup alone missed (crash, clean
exit, quiet exit inside the grace window) -- previously the tile just
stayed looking alive/thinking with zero signal. ConPtyPaneView binds a
new banner to it (message + Retry + Close); CanRetry simplifies to
"any HasExited state" since the same descriptor factory can relaunch
either kind of death. HandlerRestartCoordinator's worker-side
auto-restart path is untouched.

src/ClaudeDo.Localization/locales/de.json

@@ -348,6 +348,7 @@
     "closeSession": "Sitzung beenden",
     "conptyLaunchFailed": "Interaktive Sitzung konnte nicht geöffnet werden: {0}",
     "conptyStarting": "Sitzung wird gestartet…",
+    "conptyExited": "Sitzung beendet (Code {0}).",
     "usageWarningNotDelivered": "Die Listen-Handler-Sitzung konnte nicht vor dem Usage-Limit gewarnt werden.",
     "mergeHelperTitle": "Merge-Helfer",
     "mergeHelperHandoffTitleSuffix": " (Übergabe)",