* feat: message list refactoring and optimizations
* refactor: collapse 3 memory-limit settings into single messageLimit
- Replace memoryLimitHistorical/Viewport/ActiveSession with one `messageLimit` (default 200)
- Background trim derived automatically (limit * 0.6), not user-facing
- Fix stale historyLimit overriding current messageLimit on session reload
- Remove streaming cap that reduced fetch to VIEWPORT_MESSAGES during active streams
- Deduplicate detectTurns() call in TurnGroupingContext (reuse staticValue.turns)
- Zustand v3 migration: collapse old fields, preserve user-customized values
- Server: sanitize single messageLimit field (10-500)