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>

app/person/workload/config.ts

@@ -5,14 +5,6 @@ export type DimensionType =
   | "order_by_project_name"
   | "order_by_project_type"
 
-export const dimensionOpts = [
-  { label: "全部", value: "all" },
-  { label: "用户名", value: "order_by_user_name" },
-  { label: "用户组", value: "order_by_group_name" },
-  { label: "项目名称", value: "order_by_project_name" },
-  { label: "项目类型", value: "order_by_project_type" },
-]
-
 export type MetricColumn = {
   key: string
   title: string