Fix memory leak: use weak self in timeline observation loop

[beezly]

Summary

• TimelineViewController.listenForFocusTimelineItem() uses withObservationTracking with a recursive onChange closure to watch for focused event changes. The closure captured self strongly, meaning the observation system held the TimelineViewController alive indefinitely — even after switching rooms. Since TimelineViewController holds LiveTimeline which holds LiveRoom, the entire timeline object graph accumulated with each room switch.
• Fix: capture [weak self] in the onChange closure so the observation chain breaks when the view controller is deallocated.

Relates to #89

Verification with heap

You can verify the fix using the heap command-line tool. Get the PID of the running app with pgrep -x Mactrix or from Activity Monitor, then:

1. Open the app, select a room, run heap <pid> | grep -E 'LiveTimeline|LiveRoom' — should show 1 of each
2. Switch between ~20 rooms
3. Run the same heap command again — without the fix the counts match the number of room switches; with the fix they stay at 1

[viktorstrate]

Neat with the heap command, I didn't know that one. Thank you, for finding this! Using the app now consistently uses 500-700 MB on my machine when navigating around different timelines with images etc.