Update repository URLs to GitHub

Change all references from git.mleku.dev/mleku/plebeian-signer to
github.com/PlebeianApp/plebeian-signer for the public release.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

projects/firefox/public/manifest.json

@@ -3,7 +3,7 @@
   "name": "Plebeian Signer",
   "description": "Nostr Identity Manager & Signer",
   "version": "1.0.8",
-  "homepage_url": "https://git.mleku.dev/mleku/plebeian-signer",
+  "homepage_url": "https://github.com/PlebeianApp/plebeian-signer",
   "options_page": "options.html",
   "permissions": [
     "storage"