Phase A: clear all react-hooks/exhaustive-deps warnings via useEffectEvent or hoisting

Eliminate the four remaining missing-dependency warnings reported by
react-hooks v7. Each one was a real stale-closure or re-trigger hazard;
the fix matches the intent rather than just silencing the linter.

- src/App.tsx (line 448): wrap showSessionExpired with useEffectEvent
  (onSessionExpired) so the prepare effect doesn't re-run on every
  showSessionExpired identity change but still calls the latest
  callback. Replace the two in-effect call sites.
- src/components/furni-editor/views/FurniEditorSearchView.tsx: wrap
  the on-mount onSearch('', '', 1) call with useEffectEvent so the
  callback prop isn't a missing dependency.
- src/components/notification-center/views/bubble-layouts/
  NotificationBadgeReceivedBubbleView.tsx: wrap the
  "fetch badges only if empty on mount" check with useEffectEvent
  so badgeCodes.length isn't required as a dep (and won't re-fetch
  every count change).
- src/components/navigator/views/room-settings/
  NavigatorRoomSettingsRightsTabView.tsx: switch deps from
  roomData?.roomId to roomData (the body uses roomData.roomId after
  an early return; the linter wanted the whole object).
- src/api/ui-settings/UiSettingsContext.tsx: hoist ALL_CSS_VARS
  outside the component (it's a static constant).

After this, yarn eslint reports zero exhaustive-deps warnings across
the whole src/.

https://claude.ai/code/session_01GrR87LAqnAEyKG2ZbmQt5Q

src/components/furni-editor/views/FurniEditorSearchView.tsx

@@ -1,4 +1,4 @@
-import { FC, useCallback, useEffect, useMemo, useState } from 'react';
+import { FC, useCallback, useEffect, useEffectEvent, useMemo, useState } from 'react';
 import { Button, Column, Flex, LayoutFurniIconImageView, Text } from '../../../common';
 import { FurniItem } from '../../../hooks/furni-editor';
 
@@ -30,9 +30,11 @@ export const FurniEditorSearchView: FC<FurniEditorSearchViewProps> = props =>
     const [ sortField, setSortField ] = useState<SortField>('id');
     const [ sortDir, setSortDir ] = useState<SortDir>('asc');
 
+    const initialSearch = useEffectEvent(() => onSearch('', '', 1));
+
     useEffect(() =>
     {
-        onSearch('', '', 1);
+        initialSearch();
     }, []);
 
     const handleSearch = useCallback(() =>