refactor: remove unused dead-code exports flagged by react-doctor

Low-risk subset only — each verified to have zero external references
(tsc + eslint green); API clients, stores, icons and constants left
untouched.

- libs/util.ts: unexport `is` (still used internally by isNumber)
- components/tree-select/libs/util.ts: drop unused recursive `findLabel`
- app/person/workload/config.ts: drop unused `dimensionOpts`
- components/login/libs/session-cache.ts: drop unused `getCachedSession`
- components/setting/PageSurface.tsx: drop 3 unused layout components
  (SettingTableViewport, SettingSectionHeader, SettingFilterStack)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

components/login/libs/session-cache.ts

@@ -27,9 +27,6 @@ const getValidCachedSession = <T>(redisKey: string) => {
   return cacheEntry.data as T
 }
 
-export const getCachedSession = <T>(redisKey: string) =>
-  getValidCachedSession<T>(redisKey)
-
 export const setCachedSession = <T>(redisKey: string, data: T) => {
   sessionCache.set(redisKey, {
     data,