Desktop notifications are shown twice for unencrypted highlight messages

### Steps to reproduce

1. Use Element Web or Element Desktop on any platform with desktop notifications enabled.
2. Have another user (or a bot) send a plaintext (unencrypted room) message that triggers a highlight push rule for you — e.g. a message containing your display name (.m.rule.contains_display_name), a user mention (.m.rule.is_user_mention), or @room.
3. Keep the room out of focus.

### Outcome

Two identical OS notifications are shown for the single event. Expected: one.

The bug does not reproduce in encrypted rooms, and does not reproduce for non-highlight messages — that asymmetry is explained by the root cause below.

Root cause analysis (verified against element-web v1.12.22/v1.12.23, matrix-js-sdk v41.8.0)

Notifier.onEvent subscribes to RoomEvent.Timeline on the client. For an unencrypted event whose push actions are notify + tweaks.highlight, the client emits RoomEvent.Timeline twice for the same event:

1. The event is added to the room's live timeline → the room re-emits → client emits → toast 1.
2. During sync processing, processEventsForNotifs (matrix-js-sdk/src/sync.ts ~L1918) accumulates every event with notify && tweaks.highlight and adds it to the global notification timeline set (client.getNotifTimelineSet().addLiveEvent(...), sync.ts ~L1570)
with data.liveEvent = true. The SyncApi constructor re-emits RoomEvent.Timeline from that set to the client (sync.ts ~L239) → Notifier.onEvent runs again for the same event, all guards pass (it only filters thread-list timelines via threadListType !== null), → identical toast 2.

Encrypted events don't hit path 2: at sync-processing time they are still m.room.encrypted, push evaluation yields no highlight, so they are never added to the notification timeline set (decryption happens later, and nothing re-adds them).

### Operating system

Windows 11 Pro, build 26200.8655

### Application version

v1.12.23

Start with Notifier.onEvent in element-web and trace its RoomEvent.Timeline subscription, then inspect matrix-js-sdk/src/sync.ts around the cited notification-timeline handling and SyncApi re-emission. Reproduce the plaintext highlight case and verify that one event produces one OS notification without changing the encrypted or non-highlight behavior.

Written by the indexing model from the issue text.