refactor: convert auth register to Server Action

- Add src/app/actions/auth.ts with registerUser
- Update RegisterForm to use Server Action
- Remove api/auth/register route

src/app/actions/auth.ts

@@ -0,0 +1,23 @@
+"use server";
+
+import { AuthServerService } from "@/lib/services/auth-server.service";
+import { ERROR_CODES } from "@/constants/errorCodes";
+import { AppError } from "@/utils/errors";
+
+/**
+ * Inscrit un nouvel utilisateur
+ */
+export async function registerUser(
+  email: string,
+  password: string
+): Promise<{ success: boolean; message: string }> {
+  try {
+    await AuthServerService.registerUser(email, password);
+    return { success: true, message: "Inscription réussie" };
+  } catch (error) {
+    if (error instanceof AppError) {
+      return { success: false, message: error.message };
+    }
+    return { success: false, message: "Erreur lors de l'inscription" };
+  }
+}