feat: add doubao provider and ByteDance sponsorship (#329)

* feat: add doubao provider and ByteDance sponsorship

- Add doubao provider using DeepSeek SDK with Volcengine base URL
- Add ByteDance Doubao sponsorship acknowledgment to about pages
- Update all README files (EN/CN/JA) with K2-thinking model info
- Update ai-providers.md with doubao configuration
- Keep both gateway and doubao providers after merge

* style: auto-format with Biome

* feat: add doubao and sglang to provider config panel

* fix: add doubao and sglang to validate-model API and logo maps

* docs: update ByteDance sponsorship note in all README versions

* docs: add Doubao logo to sponsorship note

* fix: use raw GitHub URL for Doubao logo in READMEs

* fix: separate link and image in sponsorship note

* fix: use PNG instead of SVG for Doubao logo

* fix: use current branch for PNG URL (will update to main after merge)

* docs: reorganize Deployment section and update image URLs to main

---------

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

app/api/validate-model/route.ts

@@ -225,6 +225,27 @@ export async function POST(req: Request) {
                 break
             }
 
+            case "sglang": {
+                // SGLang is OpenAI-compatible
+                const sglang = createOpenAI({
+                    apiKey: apiKey || "not-needed",
+                    baseURL: baseUrl || "http://127.0.0.1:8000/v1",
+                })
+                model = sglang.chat(modelId)
+                break
+            }
+
+            case "doubao": {
+                // ByteDance Doubao uses DeepSeek-compatible API
+                const doubao = createDeepSeek({
+                    apiKey,
+                    baseURL:
+                        baseUrl || "https://ark.cn-beijing.volces.com/api/v3",
+                })
+                model = doubao(modelId)
+                break
+            }
+
             default:
                 return NextResponse.json(
                     { valid: false, error: `Unknown provider: ${provider}` },