perf: reduce UI render fanout and scroll jitter

- Cut broad render fanout across the app by replacing shared-store whole-object subscriptions with leaf selectors, memoizing hot chrome boundaries, and isolating disabled global providers from live session/message state. This keeps header controls, composer toolbars, side panels, and other non-hot UI surfaces from repainting on every assistant update or keystroke.

- Rework sidebar session ordering so recent, project groups, and worktree groups derive from one ordering source while avoiding streaming-time thrash. The sidebar now uses a stabilized session snapshot, preserves structural identity for unchanged rows, reads live row status/details per session, and applies a one-shot sort bump on idle->busy instead of continuously resorting during activity.

- Fix chat/input scroll instability by separating viewport-resize handling from message-growth handling, disabling conflicting native scroll anchoring, and stopping textarea autosize from collapsing on every growth keystroke. This removes the multiline typing jiggle during streaming and reduces unnecessary composer rerenders.

- Also gate voice context wiring behind voice-mode enablement and codify the learned render/scroll/order anti-patterns in AGENTS.md so future changes avoid the same classes of regressions.

packages/ui/src/hooks/useServerTTS.ts

@@ -125,7 +125,12 @@ export function useServerTTS(options: UseServerTTSOptions = {}): UseServerTTSRet
   const abortControllerRef = useRef<AbortController | null>(null);
   
   // Get current model, threshold, and max length from config store for summarization
-  const { currentProviderId, currentModelId, summarizeCharacterThreshold, summarizeMaxLength, openaiApiKey, settingsZenModel } = useConfigStore();
+  const currentProviderId = useConfigStore((state) => state.currentProviderId);
+  const currentModelId = useConfigStore((state) => state.currentModelId);
+  const summarizeCharacterThreshold = useConfigStore((state) => state.summarizeCharacterThreshold);
+  const summarizeMaxLength = useConfigStore((state) => state.summarizeMaxLength);
+  const openaiApiKey = useConfigStore((state) => state.openaiApiKey);
+  const settingsZenModel = useConfigStore((state) => state.settingsZenModel);
 
   // Check if server TTS is available
   const checkAvailability = useCallback(async (): Promise<boolean> => {