refactor: message list optimizations and single messageLimit setting (#409)

* 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)

packages/ui/src/lib/desktop.ts

@@ -105,10 +105,8 @@ export type DesktopSettings = {
   directoryShowHidden?: boolean;
   filesViewShowGitignored?: boolean;
 
-  // Memory limits for message viewport management
-  memoryLimitHistorical?: number;   // Default fetch limit when loading/syncing (default: 90)
-  memoryLimitViewport?: number;     // Trim target when leaving session (default: 120)
-  memoryLimitActiveSession?: number; // Trim target for active session (default: 180)
+  // Message limit — controls fetch, trim, and Load More chunk size (default: 200)
+  messageLimit?: number;
 
   // User-added skills catalogs (persisted to ~/.config/openchamber/settings.json)
   skillCatalogs?: SkillCatalogConfig[];