feat: add bring-your-own-API-key support (#186)

- Add AI provider settings to config panel (provider, model, API key, base URL)
- Support 7 providers: OpenAI, Anthropic, Google, Azure, OpenRouter, DeepSeek, SiliconFlow
- Client API keys stored in localStorage, never stored on server
- Client settings override server env vars when provided
- Skip server credential validation when client provides API key
- Bypass usage limits (request/token/TPM) when using own API key
- Add /api/config endpoint for fetching usage limits
- Add privacy notices to settings dialog, about pages, and quota toast
- Add clear settings button to reset saved API keys
- Update README files (EN/CN/JA) with BYOK documentation

Co-authored-by: dayuan.jiang <jiangdy@amazon.co.jp>

README.md

@@ -96,6 +96,8 @@ No installation needed! Try the app directly on our demo site:
 
 > Note: Due to high traffic, the demo site currently uses minimax-m2. For best results, we recommend self-hosting with Claude Sonnet 4.5 or Claude Opus 4.5.
 
+> **Bring Your Own API Key**: You can use your own API key to bypass usage limits on the demo site. Click the Settings icon in the chat panel to configure your provider and API key. Your key is stored locally in your browser and is never stored on the server.
+
 ### Run with Docker (Recommended)
 
 If you just want to run it locally, the best way is to use Docker.