feat(03-01): add loading spinners to Login, Register, and BudgetSetup submit buttons

- LoginPage: Spinner replaces button text during loading, min-w-[120px] prevents layout shift
- RegisterPage: Spinner replaces button text during loading, min-w-[120px] prevents layout shift
- BudgetSetup: Spinner replaces create button text during saving, min-w-[120px] prevents layout shift
- All buttons remain disabled during loading/saving to prevent double-submit

frontend/src/components/BudgetSetup.tsx

@@ -4,6 +4,7 @@ import { Card, CardContent, CardHeader, CardTitle, CardFooter } from '@/componen
 import { Button } from '@/components/ui/button'
 import { Input } from '@/components/ui/input'
 import { Select, SelectContent, SelectItem, SelectTrigger, SelectValue } from '@/components/ui/select'
+import { Spinner } from '@/components/ui/spinner'
 import { budgets as budgetsApi, type Budget } from '@/lib/api'
 
 interface Props {
@@ -89,8 +90,8 @@ export function BudgetSetup({ existingBudgets, onCreated, onCancel }: Props) {
       </CardContent>
       <CardFooter className="flex gap-2 justify-end">
         <Button variant="outline" onClick={onCancel}>{t('common.cancel')}</Button>
-        <Button onClick={handleCreate} disabled={saving || !name || !startDate || !endDate}>
-          {t('common.create')}
+        <Button onClick={handleCreate} disabled={saving || !name || !startDate || !endDate} className="min-w-[120px]">
+          {saving ? <Spinner /> : t('common.create')}
         </Button>
       </CardFooter>
     </Card>