feat: add reminders for tasks and calendar events (closes #13)

- DB migration #8: reminders table (entity_type, entity_id, remind_at, dismissed, created_by)
- REST API: GET /pending, GET /?entity, POST /, PATCH /:id/dismiss, DELETE
- Client polling module (reminders.js): 60s interval, toast + Browser Notification API
- Tasks: enable reminder with custom date/time in edit modal
- Calendar: reminder offset selector (at time / 15min / 1h / 1d before)
- Bell badge shows pending count; reminders auto-dismiss after 30s or on user action
- SW shell cache updated to include reminders.js + reminders.css
- 11 new DB tests covering CRUD, pending query, dismiss, upsert, cascade delete, constraints

public/index.html

@@ -38,6 +38,7 @@
   <link rel="stylesheet" href="/styles/layout.css" />
   <link rel="stylesheet" href="/styles/glass.css" />
   <link rel="stylesheet" href="/styles/login.css" />
+  <link rel="stylesheet" href="/styles/reminders.css" />
 
   <!-- Theme: Vor CSS-Rendering anwenden (Flash-Prevention) -->
   <script>