fix: a11y nested button, file_count locale formatting, scoped query key

1. PackageRow: replaced nested button>checkbox with div[role=checkbox]
   and visual-only checkbox indicator. Keyboard Enter/Space handled.
2. file_count now uses toLocaleString() matching estimated_functions
3. useUserUsage query key includes userId to prevent cache bleed
   between users on shared devices

Author: DevanshuNEU

frontend/src/components/DirectoryPicker.tsx

@@ -263,29 +263,36 @@ function PackageRow({
   onToggle: () => void
 }) {
   return (
-    <button
+    <div
+      role="checkbox"
+      aria-checked={isSelected}
+      tabIndex={0}
       onClick={onToggle}
+      onKeyDown={(e) => { if (e.key === 'Enter' || e.key === ' ') { e.preventDefault(); onToggle() } }}
       className={cn(
-        'flex items-center gap-3 w-full px-6 py-2.5 text-left transition-colors',
+        'flex items-center gap-3 w-full px-6 py-2.5 text-left transition-colors cursor-pointer',
         isSelected
           ? 'bg-primary/5'
           : 'hover:bg-muted/50',
       )}
     >
-      <Checkbox checked={isSelected} tabIndex={-1} className="pointer-events-none" />
+      <div className={cn(
+        'h-4 w-4 shrink-0 rounded-sm border',
+        isSelected ? 'bg-primary border-primary' : 'border-muted-foreground/40',
+      )} />
       <span className={cn(
         'text-sm flex-1 truncate',
         isSelected ? 'text-foreground font-medium' : 'text-muted-foreground',
       )}>
         {dir.name}
       </span>
       <span className="text-xs text-muted-foreground tabular-nums w-20 text-right">
-        {dir.file_count} files
+        {dir.file_count.toLocaleString()} files
       </span>
       <span className="text-xs text-muted-foreground tabular-nums w-24 text-right">
         ~{dir.estimated_functions.toLocaleString()} fn
       </span>
-    </button>
+    </div>
   )
 }
 

frontend/src/components/dashboard/DashboardHome.tsx

@@ -23,7 +23,7 @@ export function DashboardHome() {
   const { session } = useAuth()
   const [searchParams, setSearchParams] = useSearchParams()
   const { data: repos = [], isLoading: reposLoading, invalidate: refreshRepos } = useRepos(session?.access_token)
-  const { data: usage } = useUserUsage(session?.access_token)
+  const { data: usage } = useUserUsage(session?.access_token, session?.user?.id)
 
   const [selectedRepo, setSelectedRepo] = useState<string | null>(null)
   const [activeTab, setActiveTab] = useState<RepoTab>('overview')

frontend/src/hooks/useCachedQuery.ts

@@ -192,9 +192,9 @@ export function useRepos(apiKey: string | undefined) {
 
 
 /** User usage and tier limits from backend -- single source of truth */
-export function useUserUsage(apiKey: string | undefined) {
+export function useUserUsage(apiKey: string | undefined, userId?: string) {
   return useQuery({
-    queryKey: ['user', 'usage'],
+    queryKey: ['user', 'usage', userId],
     queryFn: async () => {
       const data = await fetchWithAuth(`${API_URL}/users/usage`, apiKey!)
       return data as {