feat(ui): add usage pill to footer and mission control header

Adds the IWorkerClient/WorkerClient usage surface (GetUsageSnapshot,
GetModelUsage, GetTaskUsage, UsageUpdated event) and a shared
UsagePillViewModel hosted once in IslandsShellViewModel (footer) and
once in MissionControlViewModel (header), showing "5h X% · 7d Y%"
with warn/blocked/stale states via existing design tokens. The
OpenMonitorCommand is wired but currently a no-op, pending the usage
monitor modal.

src/ClaudeDo.Localization/locales/de.json

@@ -562,5 +562,21 @@
     "listSettings": { "untitled": "Unbenannt" },
     "detailsIsland": { "verifyFailed": "Merge ist erfolgt, aber das Verifikationskommando der Liste ist fehlgeschlagen — die Aufgabe wurde nicht auf 'Erledigt' gesetzt." },
     "lists": { "localSuffix": "{0} / lokal", "smartMyDay": "Mein Tag", "smartImportant": "Wichtig", "smartPlanned": "Geplant", "virtualQueue": "Warteschlange", "virtualRunning": "Läuft", "virtualReview": "Prüfung", "newList": "Neue Liste" }
+  },
+  "usage": {
+    "pill": {
+      "empty": "Nutzung –",
+      "bothFormat": "5h {0}% · 7d {1}%",
+      "fiveHourFormat": "5h {0}%",
+      "sevenDayFormat": "7d {0}%",
+      "fiveHourLabel": "5h",
+      "sevenDayLabel": "7d",
+      "resetIn": "{0}: Reset in {1}",
+      "durationHoursMinutes": "{0} h {1} m",
+      "durationMinutes": "{0} m",
+      "blockedReason": "Blockiert: {0}",
+      "stale": "veraltet (Stand {0})",
+      "lastError": "Letzter Fehler: {0}"
+    }
   }
 }