Persist values, a11y errors, and aria-invalid on /register

- Server action now echoes firstName, lastName, email, and the privacy
  checkbox state on every error path so failed submits don't wipe the form.
- New `attempt` counter on RegisterState drives a remount key per field,
  bypassing React's default <form action> reset semantics.
- Error <p> elements gain role="alert" + aria-live="polite" so screen
  readers announce validation failures live.
- Privacy checkbox now flags aria-invalid when not accepted and points to
  the error message via aria-describedby.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/app/register/register-form.tsx

@@ -7,45 +7,58 @@ import { Checkbox } from "@/components/ui/checkbox";
 import { Input } from "@/components/ui/input";
 import { Label } from "@/components/ui/label";
 
-import { registerAction, type RegisterState } from "./actions";
-
-const initialState: RegisterState = {};
+import { initialRegisterState, registerAction } from "./actions";
 
 export function RegisterForm() {
-  const [state, formAction, pending] = useActionState(registerAction, initialState);
+  const [state, formAction, pending] = useActionState(
+    registerAction,
+    initialRegisterState,
+  );
+
+  // Bumping the key on each attempt forces uncontrolled inputs to remount with
+  // the echoed defaultValue from the server — otherwise React's <form action>
+  // semantics would wipe them after a failed submit.
+  const attemptKey = state.attempt;
 
   return (
     <form action={formAction} className="space-y-4">
       <div className="grid gap-4 sm:grid-cols-2">
         <FormField
+          key={`firstName-${attemptKey}`}
           id="firstName"
           name="firstName"
           label="Vorname"
           autoComplete="given-name"
           required
+          defaultValue={state.values?.firstName ?? ""}
           error={state.errors?.firstName?.[0]}
         />
         <FormField
+          key={`lastName-${attemptKey}`}
           id="lastName"
           name="lastName"
           label="Nachname"
           autoComplete="family-name"
           required
+          defaultValue={state.values?.lastName ?? ""}
           error={state.errors?.lastName?.[0]}
         />
       </div>
 
       <FormField
+        key={`email-${attemptKey}`}
         id="email"
         name="email"
         type="email"
         label="E-Mail"
         autoComplete="email"
         required
+        defaultValue={state.values?.email ?? ""}
         error={state.errors?.email?.[0]}
       />
 
       <FormField
+        key={`password-${attemptKey}`}
         id="password"
         name="password"
         type="password"
@@ -57,23 +70,51 @@ export function RegisterForm() {
       />
 
       <div className="flex items-start gap-3 pt-2">
-        <Checkbox id="acceptPrivacyPolicy" name="acceptPrivacyPolicy" required />
+        <Checkbox
+          key={`acceptPrivacyPolicy-${attemptKey}`}
+          id="acceptPrivacyPolicy"
+          name="acceptPrivacyPolicy"
+          required
+          defaultChecked={state.values?.acceptPrivacyPolicy === true}
+          aria-invalid={!!state.errors?.acceptPrivacyPolicy?.[0]}
+          aria-describedby={
+            state.errors?.acceptPrivacyPolicy?.[0]
+              ? "acceptPrivacyPolicy-error"
+              : undefined
+          }
+        />
         <div className="grid gap-1 leading-none">
           <Label htmlFor="acceptPrivacyPolicy" className="text-sm font-normal">
             Ich habe die{" "}
-            <a href="/datenschutz" className="underline" target="_blank" rel="noreferrer">
+            <a
+              href="/datenschutz"
+              className="underline"
+              target="_blank"
+              rel="noreferrer"
+            >
               Datenschutzerklärung
             </a>{" "}
             gelesen und akzeptiere sie.
           </Label>
           {state.errors?.acceptPrivacyPolicy?.[0] && (
-            <p className="text-xs text-destructive">{state.errors.acceptPrivacyPolicy[0]}</p>
+            <p
+              id="acceptPrivacyPolicy-error"
+              role="alert"
+              aria-live="polite"
+              className="text-xs text-destructive"
+            >
+              {state.errors.acceptPrivacyPolicy[0]}
+            </p>
           )}
         </div>
       </div>
 
       {state.errors?._form?.[0] && (
-        <p className="rounded-md border border-destructive/50 bg-destructive/10 p-3 text-sm text-destructive">
+        <p
+          role="alert"
+          aria-live="polite"
+          className="rounded-md border border-destructive/50 bg-destructive/10 p-3 text-sm text-destructive"
+        >
           {state.errors._form[0]}
         </p>
       )}
@@ -93,6 +134,7 @@ function FormField({
   required,
   autoComplete,
   helperText,
+  defaultValue,
   error,
 }: {
   id: string;
@@ -102,8 +144,11 @@ function FormField({
   required?: boolean;
   autoComplete?: string;
   helperText?: string;
+  defaultValue?: string;
   error?: string;
 }) {
+  const errorId = error ? `${id}-error` : undefined;
+  const helperId = !error && helperText ? `${id}-helper` : undefined;
   return (
     <div className="space-y-1.5">
       <Label htmlFor={id}>{label}</Label>
@@ -113,12 +158,23 @@ function FormField({
         type={type}
         required={required}
         autoComplete={autoComplete}
+        defaultValue={defaultValue}
         aria-invalid={!!error}
+        aria-describedby={errorId ?? helperId}
       />
       {error ? (
-        <p className="text-xs text-destructive">{error}</p>
+        <p
+          id={errorId}
+          role="alert"
+          aria-live="polite"
+          className="text-xs text-destructive"
+        >
+          {error}
+        </p>
       ) : helperText ? (
-        <p className="text-xs text-muted-foreground">{helperText}</p>
+        <p id={helperId} className="text-xs text-muted-foreground">
+          {helperText}
+        </p>
       ) : null}
     </div>
   );