implement registration-confirmation-email

- Add qrbill and pillow dependencies for Swiss QR-bill PNG generation
- Add _STRINGS_DE / _STRINGS_EN bilingual string tables to AdminNotifier
- Add _generate_qr_bill_png(): Swiss QR code with cross overlay (PNG bytes)
- Add _build_parent_html(): HTML confirmation with inline cid:qrbill image
- Add _build_parent_text(): plain-text fallback with IBAN in full
- Add notify_parent(): multipart/mixed MIME email to parent on completion
- Persist metadata.language in _build_record() in json_store.py
- Wire notify_parent() into src/agent/core.py and chat_app.py completion events
- Add 9 tests for notify_parent, _generate_qr_bill_png, language fallback
- Update tasks.md: tasks 1–5 complete; task 6 (smoke test) remains manual

112 tests passing.

https://claude.ai/code/session_01LjjK7RjKVnC8bETtccfgna

src/storage/json_store.py

@@ -254,6 +254,7 @@ class ConversationStore:
             "channel": "email",
             "parentEmail": state.parent_email,
             "conversationId": state.conversation_id,
+            "language": state.language,
         }
         return record