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/pages/RegisterPage.tsx

@@ -5,6 +5,7 @@ import { Button } from '@/components/ui/button'
 import { Card, CardContent, CardFooter, CardHeader, CardTitle } from '@/components/ui/card'
 import { Input } from '@/components/ui/input'
 import { Alert, AlertDescription } from '@/components/ui/alert'
+import { Spinner } from '@/components/ui/spinner'
 import { palette } from '@/lib/palette'
 import type { AuthContext } from '@/hooks/useAuth'
 
@@ -86,8 +87,8 @@ export function RegisterPage({ auth: { register }, onToggle }: Props) {
               required
               minLength={8}
             />
-            <Button type="submit" disabled={loading} className="w-full">
-              {t('auth.register')}
+            <Button type="submit" disabled={loading} className="w-full min-w-[120px]">
+              {loading ? <Spinner /> : t('auth.register')}
             </Button>
           </CardContent>
         </form>