feat(03-02): add domaincheck frontend - input form, results display, i18n

- DomainInput component with text input and submit button
- ResultList component with green/red status badges (D-01)
- checkDomainAction fetches POST /modules/domaincheck/check with auth cookie
- Page renders within portal AppShell at /modules/domaincheck
- i18n keys added for both DE and EN locales

apps/web/src/app/(portal)/modules/domaincheck/actions.ts

@@ -0,0 +1,46 @@
+/**
+ * Client-side action to check domain availability via the API.
+ *
+ * Calls POST /modules/domaincheck/check with the domain label.
+ * Auth cookie is forwarded via credentials: 'include'.
+ */
+
+const API_URL = process.env.NEXT_PUBLIC_API_URL || 'http://localhost:3001';
+
+export interface DomainCheckResult {
+  tld: string;
+  fqdn: string;
+  status: 'available' | 'registered';
+}
+
+export interface DomainCheckResponse {
+  domain: string;
+  results: DomainCheckResult[];
+}
+
+/**
+ * Check domain availability across TLD variants.
+ *
+ * @param domain - Base domain label (e.g., "example")
+ * @returns Response with domain and results array
+ * @throws Error if the API call fails
+ */
+export async function checkDomainAction(
+  domain: string,
+): Promise<DomainCheckResponse> {
+  const response = await fetch(`${API_URL}/modules/domaincheck/check`, {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify({ domain }),
+    credentials: 'include',
+  });
+
+  if (!response.ok) {
+    const errorBody = await response.text().catch(() => '');
+    throw new Error(
+      `Domain check failed: ${response.status} ${errorBody}`.trim(),
+    );
+  }
+
+  return response.json();
+}