perf: release timeline memory when navigating between rooms

[9clg6]

Summary

Fix memory leaks and unbounded growth in ChatController that cause increasing memory pressure during normal navigation between rooms.

Changes

• Fix timeline subscription leaks: timeline!.cancelSubscriptions() is now called BEFORE timeline = null in scrollDown(), scrollToEventId() and _reloadTimelineAndWaitForRender(). Previously, nulling the reference first left 5 orphaned stream subscriptions per timeline reload.
• Add _trimTimelineIfNeeded(): Caps timeline events at 500 per room after each requestHistory(). Prevents unbounded memory growth during long scroll sessions. Trimmed events remain in the database and are re-fetched if the user scrolls back.
• Clear Flutter image cache on dispose(): Calls PaintingBinding.instance.imageCache.clear() and clearLiveImages() when leaving a room to free decoded GPU/raster bitmaps.
• Fix timer leak: Cancel _timestampTimer in dispose().

Performance measurements

Measured with Flutter Web --profile, CPU throttling 4x, Chrome DevTools trace, ~75s navigation (DMs, groups 51-85 members, contacts, settings, scrolling, member lists). Fresh Chrome instance.

Metric	main	timeline-leaks	Delta
JS Heap delta	+73.39 MB	+21.24 MB	-71.1%
JS Heap end	170.94 MB	117.46 MB	-31.3%
Long tasks (>50ms)	51	34	-33.3%
GC events	5,538	1,669	-69.9%
GC total time	1,702 ms	923 ms	-45.8%
GC max pause	22.4 ms	16.0 ms	-28.6%

Raw trace file attached: perf-trace-timeline-leaks.json.gz (load in Chrome DevTools > Performance > Load profile to verify).

Test recommendation

1. Open a room with many messages, scroll up extensively to load history
2. Navigate to another room, repeat
3. Monitor memory in DevTools — should stay bounded instead of growing linearly
4. Verify that scrolling back to old messages still loads them from the database

Ticket

No ticket — discovered during performance profiling.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 87f43054-9c12-47f1-9cff-fe54a6ef1463

📥 Commits

Reviewing files that changed from the base of the PR and between 844b35b and fa4e9b7.

📒 Files selected for processing (1)

• lib/pages/chat/chat.dart

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch perf/timeline-memory-leaks

📝 Coding Plan

• Generate coding plan for human review comments

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[github-actions]

This PR has been deployed to https://linagora.github.io/twake-on-matrix/2949