feat(web): integrate custom providers into the provider credentials section

- Merge the separate custom-providers section into the unified provider
  grid; "Custom" in the add-provider picker now acts as an add-new action
  (trailing + mark) and opens the dedicated modal, supporting multiple
  OpenAI-compatible endpoints
- Simplify the custom provider modal: drop the default-model field, add
  an inline delete button, align colors with the theme
- Keep the legacy single "custom" card visible (models page, chat
  dropdown and legacy config page) while custom_api_key/custom_api_base
  is still in use, so existing single-provider setups don't disappear
- Unify user-facing wording from "vendor" to "provider" in UI and docs
- Restructure custom provider docs (zh/en/ja) around Web console and
  config file usage

docs/releases/v2.0.3.mdx

@@ -34,7 +34,7 @@ Related commits: [30c6d9b](https://github.com/zhayujie/CowAgent/commit/30c6d9b)
 
 ## 💰 Coding Plan Support
 
-Added integration with vendor Coding Plan (monthly programming subscription) tiers via the unified OpenAI-compatible path. Supported vendors include Aliyun, MiniMax, GLM, Kimi, and Volcengine.
+Added integration with provider Coding Plan (monthly programming subscription) tiers via the unified OpenAI-compatible path. Supported providers include Aliyun, MiniMax, GLM, Kimi, and Volcengine.
 
 See [Coding Plan docs](https://docs.cowagent.ai/en/models/coding-plan) for detailed configuration.
 

docs/releases/v2.0.7.mdx

@@ -22,7 +22,7 @@ Docs: [Image Generation Skill](https://docs.cowagent.ai/en/skills/image-generati
 - **Claude Opus 4.7**: Added `claude-opus-4-7` model support
 - **GLM 5.1**: Added `glm-5.1` model support
 - **Kimi Coding Plan**: Support for Kimi Coding Plan mode
-- **Custom model providers**: New custom model provider configuration for easier integration with additional vendors
+- **Custom model providers**: New custom model provider configuration for easier integration with additional providers
 
 ## 💬 Web Console Improvements