feat: push notifications on each device (Web Push)

- What reaches the bell also reaches the devices where the person
  turned «Avisos en este dispositivo» on, with TaskStream closed: the
  Notifier sends each new notification encrypted (aes128gcm) and signed
  with VAPID, and forgets subscriptions the push service dropped
  (404/410). A failure never blocks the event stream.
- The API keeps one subscription per device (push_subscriptions) and
  exposes the VAPID public key in the notification settings; GraphQL
  subscribePush/unsubscribePush, REST /me/push-subscriptions. Without
  VAPID_PUBLIC_KEY and VAPID_PRIVATE_KEY push is off.
- The web registers /push-sw.js, a service worker that only shows the
  notification and opens its ticket on a tap (no caching, served
  without cache by nginx), and a manifest that makes it installable,
  which iPhones need for push.
- The bell's toggles show their text next to them.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

libs/api/tracker/presenters/src/lib/http/dto/responses.dto.ts

@@ -844,4 +844,11 @@ export class NotificationSettingsResponseDto {
   @ApiProperty({ description: 'Copies by e-mail' }) email!: boolean;
   @ApiProperty({ description: 'Whether this installation can send e-mail' })
   emailAvailable!: boolean;
+  @ApiProperty({
+    type: String,
+    nullable: true,
+    description:
+      'VAPID public key to subscribe a device to push with; null: no push here',
+  })
+  pushPublicKey!: string | null;
 }