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/agent/core.py

@@ -129,6 +129,13 @@ class EmailAgent:
                 )
             except Exception:
                 logger.exception("Failed to send admin notification for %s", email_key)
+            try:
+                self._notifier.notify_parent(
+                    registration=state.registration,
+                    language=state.language,
+                )
+            except Exception:
+                logger.exception("Failed to send parent confirmation for %s", email_key)
             logger.info("Registration complete for %s", state.conversation_id)
 
         return reply_text