refactor: remove legacy frontend code and implement new Next.js structure

- Deleted the old Register page and utility functions.
- Removed Tailwind CSS configuration and Vite configuration files.
- Added a new script for starting a single container with FastAPI and Next.js.
- Updated README to reflect the current status of the Next.js frontend.
- Implemented new login and registration API routes with improved error handling.
- Refactored frontend API calls to use the new proxy structure.
- Enhanced error handling in API response processing.
- Updated components to align with the new API endpoints and structure.

web/src/lib/api/response.ts

@@ -0,0 +1,61 @@
+function isJsonContentType(contentType: string | null) {
+  return Boolean(contentType && (contentType.includes("application/json") || contentType.includes("+json")));
+}
+
+export function isRecord(value: unknown): value is Record<string, unknown> {
+  return typeof value === "object" && value !== null;
+}
+
+export async function readResponsePayload(response: Response): Promise<unknown> {
+  if (response.status === 204 || response.status === 205) {
+    return null;
+  }
+
+  const raw = await response.text();
+  if (!raw) {
+    return null;
+  }
+
+  if (!isJsonContentType(response.headers.get("content-type"))) {
+    return raw;
+  }
+
+  try {
+    return JSON.parse(raw) as unknown;
+  } catch {
+    return raw;
+  }
+}
+
+export function getResponseErrorMessage(payload: unknown, fallback: string) {
+  if (isRecord(payload)) {
+    const detail = payload.detail;
+    if (typeof detail === "string" && detail.trim()) {
+      return detail;
+    }
+
+    const message = payload.message;
+    if (typeof message === "string" && message.trim()) {
+      return message;
+    }
+  }
+
+  if (typeof payload === "string" && payload.trim()) {
+    const trimmed = payload.trim();
+    if (trimmed.startsWith("<!DOCTYPE") || trimmed.startsWith("<html")) {
+      return `${fallback} 接口返回了 HTML 而不是 JSON，请检查前端代理和后端服务。`;
+    }
+    return trimmed;
+  }
+
+  return fallback;
+}
+
+export function getUnexpectedJsonMessage(response: Response) {
+  const contentType = response.headers.get("content-type") || "unknown content type";
+  if (contentType.includes("text/html")) {
+    return "接口返回了 HTML 而不是 JSON，请检查前端代理和后端服务。";
+  }
+
+  return `接口返回了非 JSON 响应：${contentType}`;
+}