feat(03-02): add EmptyState component and wire into Dashboard and Categories pages

- Create shared EmptyState component with icon, heading, subtext, and optional CTA button
- DashboardPage: show EmptyState when no budgets exist with Create CTA; replace plain Card fallback with EmptyState for no-current-budget case
- CategoriesPage: add loading state to prevent empty-state flash on initial load; show EmptyState when no categories exist

frontend/src/components/EmptyState.tsx

@@ -0,0 +1,21 @@
+import { Button } from '@/components/ui/button'
+
+interface EmptyStateProps {
+  icon: React.ElementType
+  heading: string
+  subtext: string
+  action?: { label: string; onClick: () => void }
+}
+
+export function EmptyState({ icon: Icon, heading, subtext, action }: EmptyStateProps) {
+  return (
+    <div className="flex flex-col items-center justify-center py-16 text-center">
+      <Icon className="size-12 text-muted-foreground mb-4" />
+      <h2 className="font-semibold text-lg mb-1">{heading}</h2>
+      <p className="text-sm text-muted-foreground mb-4">{subtext}</p>
+      {action && (
+        <Button onClick={action.onClick}>{action.label}</Button>
+      )}
+    </div>
+  )
+}