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/tree-select/libs/util.ts

@@ -75,24 +75,6 @@ export const buildTreeValueMap = (
   return acc
 }
 
-export const findLabel = (
-  nodes: TreeNodeData[],
-  targetValue: string,
-  pathArr: string[] = [] // 内部用，调用端无需传入
-): FindResult => {
-  for (const node of nodes) {
-    const curPath = [...pathArr, node.label as string] // 复制一份，避免污染上层
-    if (node.value === targetValue) {
-      return { path: curPath.join(TREE_PATH_SEPARATOR), node }
-    }
-    if (node.children) {
-      const found = findLabel(node.children, targetValue, curPath)
-      if (found.node) return found
-    }
-  }
-  return { path: "", node: null }
-}
-
 export const findNodesByValues = (
   valueMap: Record<string, TreeValueMeta>,
   targetValues: string[]