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

server/db-schema-test.js

@@ -178,6 +178,20 @@ const MIGRATIONS_SQL = {
     );
     CREATE INDEX IF NOT EXISTS idx_calendar_external_id ON calendar_events(external_calendar_id);
   `,
+  8: `
+    CREATE TABLE IF NOT EXISTS reminders (
+      id          INTEGER PRIMARY KEY AUTOINCREMENT,
+      entity_type TEXT    NOT NULL CHECK(entity_type IN ('task', 'event')),
+      entity_id   INTEGER NOT NULL,
+      remind_at   TEXT    NOT NULL,
+      dismissed   INTEGER NOT NULL DEFAULT 0,
+      created_by  INTEGER NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+      created_at  TEXT    NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%SZ', 'now'))
+    );
+    CREATE INDEX IF NOT EXISTS idx_reminders_entity ON reminders(entity_type, entity_id);
+    CREATE INDEX IF NOT EXISTS idx_reminders_remind ON reminders(remind_at);
+    CREATE INDEX IF NOT EXISTS idx_reminders_user   ON reminders(created_by);
+  `,
 };
 
 export { MIGRATIONS_SQL };