feat(ui): External-Agent-API + Besitzübertragung UI (UI-Backlog Module 15+16/16)

Modul 15 External-Agent (ai_assistant-Plugin, Manifest settings_page):
- SettingsExternalAgents.tsx: Agentenliste mit curl-Snippets (run/status/stream),
  Copy-Buttons, Bearer-Token-Hinweis, Rate-Limit-Doku, Token-Link
- api/externalAgent.ts (useAiAgents, buildCurlSnippets, curlCommand)
- Manifest: settings_pages +external-agents (order 61, permission ai:read)
- Komponenten-Map regeneriert (43)

Modul 16 Ownership-Transfer (Core):
- SettingsOwnership.tsx: Admin-Gate, From/To-User-Selects, 10 Entity-Type-Chips,
  ConfirmDialog, Ergebnis-Tabelle
- api/ownership.ts (useTransferOwnership, OWNERSHIP_ENTITY_TYPES)
- Route /settings/ownership + Nav-Eintrag

i18n de/en +28 Keys. Vitest 12/12, tsc 0, Build OK, ruff OK, Manifest-Import OK

app/plugins/builtins/ai_assistant/plugin.py

@@ -59,6 +59,7 @@ class AIAssistantPlugin(BasePlugin):
         ],
         settings_pages=[
             FrontendSettingsPage(path='ai', label_key='settings.ai', label='AI Settings', component='@/pages/AISettings', icon='Bot', order=60),
+            FrontendSettingsPage(path='external-agents', label_key='settings.externalAgents', label='External Agents API', component='@/pages/SettingsExternalAgents', icon='Bot', order=61, permission='ai:read'),
         ],
         author="LeoCRM Team",
         min_app_version="1.0.0",