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/config/route.ts

@@ -0,0 +1,10 @@
+import { NextResponse } from "next/server"
+
+export async function GET() {
+    return NextResponse.json({
+        accessCodeRequired: !!process.env.ACCESS_CODE_LIST,
+        dailyRequestLimit: Number(process.env.DAILY_REQUEST_LIMIT) || 0,
+        dailyTokenLimit: Number(process.env.DAILY_TOKEN_LIMIT) || 0,
+        tpmLimit: Number(process.env.TPM_LIMIT) || 0,
+    })
+}