feat(frontend): add typed API contract layer

Made-with: Cursor

frontend/src/api/http.ts

@@ -0,0 +1,55 @@
+import { ApiError, toApiError } from "./errors";
+
+const API_BASE_URL = (import.meta.env.VITE_API_BASE_URL as string | undefined)?.trim() || "http://localhost:3001";
+
+function buildUrl(path: string): string {
+  const normalizedPath = path.startsWith("/") ? path : `/${path}`;
+  return `${API_BASE_URL}${normalizedPath}`;
+}
+
+async function parseResponseBody(response: Response): Promise<unknown> {
+  const contentType = response.headers.get("content-type") ?? "";
+  if (!contentType.includes("application/json")) {
+    return null;
+  }
+
+  try {
+    return await response.json();
+  } catch {
+    return null;
+  }
+}
+
+export async function requestJson<T>(path: string, init?: RequestInit): Promise<T> {
+  try {
+    const response = await fetch(buildUrl(path), {
+      ...init,
+      headers: {
+        "Content-Type": "application/json",
+        ...(init?.headers ?? {}),
+      },
+    });
+
+    if (response.status === 204) {
+      return undefined as T;
+    }
+
+    const payload = await parseResponseBody(response);
+
+    if (!response.ok) {
+      throw toApiError(response.status, payload);
+    }
+
+    return payload as T;
+  } catch (error) {
+    if (error instanceof ApiError) {
+      throw error;
+    }
+
+    throw new ApiError({
+      code: "network_error",
+      status: null,
+      message: "Не удалось связаться с сервером.",
+    });
+  }
+}