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>

env.example

@@ -72,6 +72,10 @@ AI_MODEL=global.anthropic.claude-sonnet-4-5-20250929-v1:0
 # SGLANG_API_KEY=your-sglang-api-key
 # SGLANG_BASE_URL=http://127.0.0.1:8000/v1  # Your SGLang endpoint
 
+# ByteDance Doubao Configuration (via Volcengine)
+# DOUBAO_API_KEY=your-doubao-api-key
+# DOUBAO_BASE_URL=https://ark.cn-beijing.volces.com/api/v3  # ByteDance Volcengine endpoint
+
 # Vercel AI Gateway Configuration
 # Get your API key from: https://vercel.com/ai-gateway
 # Model format: "provider/model" e.g., "openai/gpt-4o", "anthropic/claude-sonnet-4-5"