feat(docs-site): add starlight-plugin-icons, use real Gitea logo in header

The header social link used Starlight's generic `code-branch` icon.
Wires up starlight-plugin-icons + UnoCSS (Iconify) and overrides
SocialIcons to render `pajamas:gitea` for the Gitea link instead, while
falling back to Starlight's default icon set for anything else. Sidebar
and codeblock icon support are enabled but unused for now.

docs/site/package.json

@@ -12,9 +12,13 @@
   "dependencies": {
     "@astrojs/starlight": "^0.41.4",
     "@catppuccin/starlight": "^2.1.0",
+    "@iconify-json/pajamas": "^1.2.15",
+    "@unocss/astro": "^66.7.5",
     "astro": "^7.0.2",
     "sharp": "^0.35.3",
     "starlight-links-validator": "^0.25.2",
-    "starlight-llms-txt": "^0.11.0"
+    "starlight-llms-txt": "^0.11.0",
+    "starlight-plugin-icons": "^1.1.6",
+    "unocss": "^66.7.5"
   }
 }