fix(formatters): use raster PNG forge icons Discord renders

- GitHub footer icon now uses fluidicon.png instead of favicon.ico —
  Discord silently drops .ico embed footer icons (png/jpg/webp/gif only)
- Gitea instances use {origin}/assets/img/favicon.png (verified on
  codeberg.org and gitea.com); links are still derived from the repo URL
- tests + docs updated to the new icon URLs

docs/guide/groups.md

@@ -71,7 +71,7 @@ A group defines the forges it receives events from via `forgeSources`, a list of
 }
 ```
 
-- **Discord** — the embed footer shows the label next to the repo (`内网 Gitea · acme/widget`) with the site's favicon as the footer icon (derived from the repository URL).
+- **Discord** — the embed footer shows the label next to the repo (`内网 Gitea · acme/widget`) with a site icon as the footer icon: GitHub's `fluidicon.png` or the Gitea instance's `/assets/img/favicon.png` (both raster PNGs — Discord silently ignores `.ico` favicons).
 - **Telegram** — the footer line starts with the hyperlinked label (`[内网 Gitea](https://git1.example.com)`).
 - Events whose repository host has no matching entry (e.g. custom webhooks, or a repo hosted elsewhere) get no label.
 

docs/zh/guide/groups.md

@@ -71,7 +71,7 @@
 }
 ```
 
-- **Discord** — embed 底部在仓库名旁显示标识（`内网 Gitea · acme/widget`），并以按仓库 URL 推导的站点 favicon 作为底部图标。
+- **Discord** — embed 底部在仓库名旁显示标识（`内网 Gitea · acme/widget`），并以站点图标作为底部图标：GitHub 使用 `fluidicon.png`，Gitea 实例使用 `/assets/img/favicon.png`（均为 Discord 可渲染的栅格 PNG——`.ico` favicon 会被 Discord 静默忽略）。
 - **Telegram** — 底部行以带超链接的标识开头（`[内网 Gitea](https://git1.example.com)`）。
 - 仓库主机没有匹配条目的事件（如自定义 webhook，或仓库托管在别处）不显示标识。