fix: prevent [object Object] rendering in 25 frontend components

Replace direct {error} JSX rendering with typeof check + .message fallback.
When error is an object (not a string), React showed [object Object].
Now renders error.message or fallback string.

frontend/src/components/ui/Input.tsx

@@ -43,7 +43,7 @@ export const Input = React.forwardRef<HTMLInputElement, InputProps>(
         />
         {error && (
           <p id={errorId} className="mt-1 text-sm text-danger-600" role="alert">
-            {error}
+            {typeof error === "string" ? error : (error as any)?.message || "Ein Fehler ist aufgetreten"}
           </p>
         )}
         {helperText && !error && (

frontend/src/components/ui/Select.tsx

@@ -54,7 +54,7 @@ export const Select = React.forwardRef<HTMLSelectElement, SelectProps>(
         </select>
         {error && (
           <p id={errorId} className="mt-1 text-sm text-danger-600" role="alert">
-            {error}
+            {typeof error === "string" ? error : (error as any)?.message || "Ein Fehler ist aufgetreten"}
           </p>
         )}
         {helperText && !error && (