feat: add custom AI Studio app URL configuration

[bbbugg]

This pull request introduces support for configuring a custom AI Studio App URL, allowing users to specify their own app instead of the built-in default. It adds environment variable and documentation support for this feature, validates the provided URL, and updates the startup log to display the active app URL. Additionally, new guides (in both English and Chinese) are included to help users create and configure their own AI Studio App.
此拉取请求引入了对配置自定义 AI Studio 应用 URL 的支持，允许用户指定自己的应用，而非使用内置的默认应用。该功能增加了对环境变量和文档的支持，验证了所提供的 URL，并更新了启动日志以显示当前生效的应用 URL。此外，还包含了新的指南（中英文版本），旨在帮助用户创建并配置其专属的 AI Studio 应用。

• close Allow configuring the AI Studio Build App URL #266
• close Make targetUrl configurable via environment variable #92
• Related to fix: 403 PERMISSION_DENIED - add OAuth token from gapi.auth.getToken() to fetch calls #232
• Related to [反馈]: Codex 调用 Banana 2 和 Pro 时频繁触发 429 Token 限额 #246

[Copilot]

🟡 Changes recommended

The critical logging issue must be fixed, and the environment-file documentation should be clarified.

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Pull request overview

Adds configurable custom AI Studio App URLs with validation, fallback behavior, startup logging, and bilingual documentation.

Changes:

• Adds AI_STUDIO_APP_URL configuration and validation.
• Uses the configured URL in BrowserManager.
• Updates environment examples, READMEs, and setup guides.

File summaries

File	Summary
src/utils/ConfigLoader.js	Critical (1 vote): rejected environment values may expose credentials or inject log lines; log only a generic or sanitized validation failure.
src/core/BrowserManager.js	Uses the configured AI Studio App URL for navigation.
README.md	Documents the custom URL configuration in Chinese.
README_EN.md	Documents the custom URL configuration in English.
docs/zh/create-ai-studio-app.md	Nit (1 vote): document both .env and .env.development for development usage and restart instructions.
docs/en/create-ai-studio-app.md	Nit (1 vote): document both .env and .env.development for development usage and restart instructions.
.env.example	Adds the AI_STUDIO_APP_URL example configuration.

Review details

Suppressed comments (2)

docs/en/create-ai-studio-app.md:65

• This guide only tells users to write .env, but main.js loads .env.development for non-production runs (including the documented npm run dev path). Following this guide in development therefore leaves AI_STUDIO_APP_URL unset and silently uses the built-in app; document both environment files here and in the restart instruction.

Add the complete link to the `.env` file in the project root:

```env
AI_STUDIO_APP_URL=https://ai.studio/apps/xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx

docs/zh/create-ai-studio-app.md:65

• 本教程只要求写入 .env，但 main.js 在非生产环境（包括文档中的 npm run dev）会读取 .env.development。用户按此步骤在开发环境配置时，AI_STUDIO_APP_URL 会被忽略并静默使用内置应用；这里以及后面的重启说明应同时写明两个环境文件。

将完整链接写入项目根目录的 `.env`：

```env
AI_STUDIO_APP_URL=https://ai.studio/apps/xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx

• Files reviewed: 7/12 changed files
• Comments generated: 1
• Review effort level: Lite

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