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>

libs/util.ts

@@ -1,7 +1,7 @@
 /**
  * @description: 判断值是否为某个类型
  */
-export function is(val: unknown, type: string) {
+function is(val: unknown, type: string) {
   return toString.call(val) === `[object ${type}]`
 }