Add XUI.ONE 1.5.13 installer with self-hosted Gitea mirror

Bootstrap installer that downloads the pre-patched XUI 1.5.13 package
from our Gitea generic-package mirror (GitHub fallback), checksum-verifies
it, and runs the bundled ./install. Includes preflight checks (root,
Ubuntu 20/22/24, x86_64, RAM/disk, re-install guard).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,9 @@
+# Large release archive — host on CDN / private git LFS, don't commit the 661MB zip
+release/*.zip
+release/download.log
+
+# Keep the checksum so installs can be integrity-verified
+!release/*.sha256
+
+# Gitea credentials — never commit
+.gitea/