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>

app/api/chat/route.ts

@@ -199,8 +199,17 @@ async function handleChatRequest(req: Request): Promise<Response> {
     }
     // === CACHE CHECK END ===
 
-    // Get AI model from environment configuration
-    const { model, providerOptions, headers, modelId } = getAIModel()
+    // Read client AI provider overrides from headers
+    const clientOverrides = {
+        provider: req.headers.get("x-ai-provider"),
+        baseUrl: req.headers.get("x-ai-base-url"),
+        apiKey: req.headers.get("x-ai-api-key"),
+        modelId: req.headers.get("x-ai-model"),
+    }
+
+    // Get AI model with optional client overrides
+    const { model, providerOptions, headers, modelId } =
+        getAIModel(clientOverrides)
 
     // Check if model supports prompt caching
     const shouldCache = supportsPromptCaching(modelId)