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

pyproject.toml

@@ -11,6 +11,8 @@ dependencies = [
     # Registration schema validation
     "jsonschema>=4.23.0",
     "chainlit>=2.9.6",
+    "qrbill>=1.2.0",
+    "pillow>=12.1.1",
 ]
 
 [project.scripts]