perf(ui-tui): narrow overlay subscriptions to focused selectors

Subscribe overlay components to computed theme/session selectors instead of the full UI store so unrelated UI state updates trigger fewer overlay renders.

ui-tui/src/app/uiStore.ts

@@ -1,4 +1,4 @@
-import { atom } from 'nanostores'
+import { atom, computed } from 'nanostores'
 
 import { MOUSE_TRACKING } from '../config/env.js'
 import { ZERO } from '../domain/usage.js'
@@ -30,6 +30,9 @@ const buildUiState = (): UiState => ({
 
 export const $uiState = atom<UiState>(buildUiState())
 
+export const $uiTheme = computed($uiState, state => state.theme)
+export const $uiSessionId = computed($uiState, state => state.sid)
+
 export const getUiState = () => $uiState.get()
 
 export const patchUiState = (next: Partial<UiState> | ((state: UiState) => UiState)) =>