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/components/providers/ThemeProvider.tsx

@@ -6,7 +6,10 @@ interface ThemeProviderProps {
 }
 
 export const ThemeProvider: React.FC<ThemeProviderProps> = ({ children }) => {
-  const { fontSize, applyTypography, padding, applyPadding } = useUIStore();
+  const fontSize = useUIStore((state) => state.fontSize);
+  const applyTypography = useUIStore((state) => state.applyTypography);
+  const padding = useUIStore((state) => state.padding);
+  const applyPadding = useUIStore((state) => state.applyPadding);
 
   React.useLayoutEffect(() => {
     applyTypography();