feat(T06): Mietkatalog frontend – equipment list, detail, search/filter, SSR

- composables/useApi.ts: base API client with runtimeConfig
- composables/useEquipment.ts: equipment API wrapper (list, detail, categories)
- components/EquipmentCard.vue: card with image/placeholder, badge, name, price, add-to-cart
- pages/mietkatalog.vue: SSR list with search, category chips, sort, pagination, skeleton, empty/error states
- pages/mietkatalog/[id].vue: SSR detail with specs table, related items, breadcrumb, add-to-cart
- nuxt.config.ts: runtimeConfig with apiBase
- 101 vitest tests passing (5 files)
- Build: 0 errors, 2.09 MB

frontend/composables/useApi.ts

@@ -0,0 +1,57 @@
+import type { $Fetch } from "nitropack";
+
+/**
+ * Base API client composable.
+ * Wraps $fetch with the configured API base URL from runtime config.
+ * Provides typed GET, POST, PUT, DELETE helpers.
+ */
+export function useApi() {
+  const config = useRuntimeConfig();
+  const apiBase = config.public.apiBase;
+
+  /**
+   * Typed $fetch wrapper bound to API base URL.
+   */
+  const client: $Fetch = $fetch.create({
+    baseURL: apiBase,
+    headers: {
+      "Content-Type": "application/json",
+    },
+  });
+
+  async function get<T>(url: string, params?: Record<string, string | number | boolean | undefined>): Promise<T> {
+    return client<T>(url, {
+      method: "GET",
+      params: params as Record<string, string> | undefined,
+    });
+  }
+
+  async function post<T>(url: string, body?: unknown): Promise<T> {
+    return client<T>(url, {
+      method: "POST",
+      body,
+    });
+  }
+
+  async function put<T>(url: string, body?: unknown): Promise<T> {
+    return client<T>(url, {
+      method: "PUT",
+      body,
+    });
+  }
+
+  async function del<T>(url: string): Promise<T> {
+    return client<T>(url, {
+      method: "DELETE",
+    });
+  }
+
+  return {
+    apiBase,
+    client,
+    get,
+    post,
+    put,
+    del,
+  };
+}