fix: use absolute backend URL in tauri production mode (welcome page stuck on 'Loading Failed')

[zhuohoudeputao]

问题 / Problem

On Linux (Arch + Hyprland, tauri webview), the welcome page is stuck on a "Loading Failed / 加载失败" error dialog and the main UI is unreachable. Reported as MaaEnd/MaaEnd#4692.

Root cause: getApiBase() returns the relative path /api whenever window.location.host is truthy — which is always the case under tauri://localhost. In tauri production the webview cannot reach /api (tauri does not proxy it to the axum web server), so the interface fetch fails and content loading (DESCRIPTION.md etc.) falls back to showing the raw path with the error dialog.

修复 / Fix

• getApiBase(): once the backend port has been discovered via setBackendPort(), prefer the absolute http://host:port/api URL. The relative /api path remains as a dev-mode (Vite proxy) / same-origin reverse-proxy fallback.
• getWsUrl(): same fix for the WebSocket endpoint (serverPort already exists).

Verified: pnpm build passes; the compiled getApiBase now returns the absolute URL when the backend port is known.

验证 / Verification

• Backend API on port 12701 responds correctly (curl http://127.0.0.1:12701/api/interface and /api/local-file?path=... both return 200).
• The bug is that the frontend never uses that port because the relative /api path resolves under tauri://localhost and returns the embedded index.html (SPA fallback) instead of JSON.
• After this fix, once the port-probe succeeds, all subsequent API calls go to http://localhost:12701/api/... which works.

Summary by Sourcery

确保在探测到后端端口可用时，前端使用绝对后端 URL，以修复 Tauri 生产模式下的 API 加载失败问题。

New Features:

• 当已发现后端端口时，支持通过绝对 http URL 直接连接到后端。

Bug Fixes:

• 通过避免在 Tauri 生产模式下无法访问的相对 /api 请求，修复欢迎页加载失败的问题。
• 当已知后端服务器端口时，通过优先使用绝对 ws URL 修复 WebSocket 连接问题。

Enhancements:

• 优化后端 URL 选择逻辑，更好地区分开发模式下使用代理与直接访问后端的路径。

Original summary in English

Summary by Sourcery

Ensure frontend uses an absolute backend URL when a probed backend port is available to fix API loading failures in Tauri production mode.

New Features:

• Support direct connections to the backend via an absolute http URL when the backend port has been discovered.

Bug Fixes:

• Fix welcome page failing to load by avoiding relative /api requests that are unreachable in Tauri production mode.
• Fix WebSocket connections by preferring an absolute ws URL when the backend server port is known.

Enhancements:

• Refine backend URL selection logic to better distinguish between dev-mode proxy usage and direct backend access paths.

[sourcery-ai]

Hey - 我在这里给出了一些总体反馈：

• 在 getApiBase 和 getWsUrl 中用于推导协议/主机名的逻辑是重复的，并且与 Nginx/host 分支存在一些差异；建议把 URL 构建集中到一个辅助方法中，以保持行为一致并更易于维护。
• 在 getApiBase 中，backendPort > 0 分支即使在同源反向代理后面运行时，也会始终优先选择直连 URL；如果这是刻意设计的，可能值得显式防范 hostname 与后端绑定不匹配的情况（例如，当探测到的端口只监听在 localhost 时强制使用 localhost）。

给 AI Agent 的提示

请根据这次代码评审中的评论进行修改：

## 总体评论
- 在 `getApiBase` 和 `getWsUrl` 中用于推导协议/主机名的逻辑是重复的，并且与 Nginx/host 分支存在一些差异；建议把 URL 构建集中到一个辅助方法中，以保持行为一致并更易于维护。
- 在 `getApiBase` 中，`backendPort > 0` 分支即使在同源反向代理后面运行时，也会始终优先选择直连 URL；如果这是刻意设计的，可能值得显式防范 `hostname` 与后端绑定不匹配的情况（例如，当探测到的端口只监听在 localhost 时强制使用 `localhost`）。

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Original comment in English

Hey - I've left some high level feedback:

• The logic for deriving protocol/hostname in both getApiBase and getWsUrl is duplicated and slightly different from the Nginx/host branch; consider centralizing URL construction in a helper to keep behavior consistent and easier to maintain.
• In getApiBase, the backendPort > 0 branch always prefers a direct URL even when running behind a same-origin reverse proxy; if that’s intentional, it may be worth explicitly guarding against mismatched hostname/backend binding (e.g., forcing localhost when the probed port only listens there).

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The logic for deriving protocol/hostname in both `getApiBase` and `getWsUrl` is duplicated and slightly different from the Nginx/host branch; consider centralizing URL construction in a helper to keep behavior consistent and easier to maintain.
- In `getApiBase`, the `backendPort > 0` branch always prefers a direct URL even when running behind a same-origin reverse proxy; if that’s intentional, it may be worth explicitly guarding against mismatched `hostname`/backend binding (e.g., forcing `localhost` when the probed port only listens there).

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