feat: Add Decap CMS for Meg/Holly content editing

- admin/index.html: Decap CMS loader
- admin/config.yml: Gitea backend config (native support!)
- .eleventy.js: Added admin passthrough copy

OAuth App created in Gitea:
- Client ID: ad439d72-e724-4f88-ad24-a1187c52b313
- Redirect URI: https://firefrostgaming.com/admin/

Gemini's architecture: Decap → Gitea (direct) → GitHub mirror → Cloudflare Pages
No bidirectional sync issues, Gitea remains source of truth.

Signed-off-by: Claude <claude@firefrostgaming.com>

.eleventy.js

@@ -3,6 +3,9 @@ module.exports = function(eleventyConfig) {
   // Without this, your styles and images won't appear in the _site output
   eleventyConfig.addPassthroughCopy("src/css");
   eleventyConfig.addPassthroughCopy("assets");
+  
+  // Decap CMS admin panel
+  eleventyConfig.addPassthroughCopy("admin");
 
   return {
     dir: {