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/setting/PageSurface.tsx

@@ -386,12 +386,6 @@ export function SettingCustomPageSizeControl({
   )
 }
 
-type SettingTableViewportProps = PropsWithChildren<{
-  maxHeight: number | string
-  className?: string
-  style?: CSSProperties
-}>
-
 export function SettingPage({
   className,
   ...props
@@ -428,21 +422,6 @@ export function SettingPanel({
   )
 }
 
-export function SettingTableViewport({
-  maxHeight,
-  className,
-  style,
-  children,
-}: SettingTableViewportProps) {
-  return (
-    <div
-      style={{ maxHeight, ...style }}
-      className={mergeClassName(classes.tableViewport, className)}>
-      {children}
-    </div>
-  )
-}
-
 export function SettingFilterPanel({
   className,
   ...props
@@ -467,43 +446,6 @@ export function SettingContentPanel({
   )
 }
 
-type SettingSectionHeaderProps = GroupProps & {
-  eyebrow?: string
-  title: ReactNode
-  description?: ReactNode
-  actions?: ReactNode
-}
-
-export function SettingSectionHeader({
-  eyebrow,
-  title,
-  description,
-  actions,
-  className,
-  ...props
-}: SettingSectionHeaderProps) {
-  return (
-    <Group
-      justify="space-between"
-      align="flex-start"
-      gap="sm"
-      wrap="wrap"
-      className={mergeClassName(classes.sectionHeader, className)}
-      {...props}>
-      <Stack gap={2}>
-        {eyebrow ? (
-          <Text className={classes.sectionEyebrow}>{eyebrow}</Text>
-        ) : null}
-        <Text className={classes.sectionTitle}>{title}</Text>
-        {description ? (
-          <Text className={classes.sectionDescription}>{description}</Text>
-        ) : null}
-      </Stack>
-      {actions}
-    </Group>
-  )
-}
-
 type SettingListHeaderProps = {
   title: ReactNode
   count?: ReactNode
@@ -569,20 +511,6 @@ export function SettingFilterActions({
   )
 }
 
-export function SettingFilterStack({
-  className,
-  gap = 12,
-  ...props
-}: PropsWithChildren<StackProps>) {
-  return (
-    <Stack
-      gap={gap}
-      className={mergeClassName(classes.filterStack, className)}
-      {...props}
-    />
-  )
-}
-
 type SettingInlineFilterFieldProps = {
   label: ReactNode
   children: ReactNode