fix(mobile): reach the true bottom of long chats

[cha1latte]

Summary

• keep retrying a requested jump while virtualized chat rows continue measuring
• invalidate provisional bottom observations when content size grows
• stop only after the measured bottom remains stable, with a bounded retry budget

Verification

• Expo tests: 17 passed
• controller regression tests: 3 passed
• Expo TypeScript check: passed
• new controller and regression tests pass Biome