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

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

components/model-selector.tsx

@@ -46,7 +46,9 @@ const PROVIDER_LOGO_MAP: Record<string, string> = {
     openrouter: "openrouter",
     deepseek: "deepseek",
     siliconflow: "siliconflow",
+    sglang: "openai", // SGLang is OpenAI-compatible, use OpenAI logo
     gateway: "vercel",
+    doubao: "bytedance",
 }
 
 // Group models by providerLabel (handles duplicate providers)