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>

components/quota-limit-toast.tsx

@@ -82,9 +82,9 @@ export function QuotaLimitToast({
                     </Link>
                 </p>
                 <p>
-                    The good news is that you can self-host the project in
-                    seconds on Vercel (it's fully open-source), or if you love
-                    it, consider sponsoring to help keep the lights on!
+                    <strong>Tip:</strong> You can use your own API key (click
+                    the Settings icon) or self-host the project to bypass these
+                    limits.
                 </p>
                 <p>Your limit resets tomorrow. Thanks for understanding!</p>
             </div>