chore: migrate self-distribution links from GitHub to self-hosted Gitea

GitHub accounts suspended; all first-party distribution now served from
git.881218.xyz/rogue/NexusGate (Gitea):
- raw:  /rogue/NexusGate/raw/branch/<branch>/<path>
- release assets and source archives under the same repo path
- scripts gain a GIT_BASE variable built from NG_REPO (default
  rogue/NexusGate); NG_REPO/NG_BRANCH/--repo/--branch semantics unchanged
- panel enrollment/uninstall commands in public/app.js point at Gitea raw
- install.sh/nexusgate.sh/agent-singbox.sh stats source previously
  defaulted to the upstream repo (a2899882); defaults now this repo
Upstream dependency references (sagernet/sing-box Go module) untouched.

scripts/install.sh

@@ -1,8 +1,10 @@
 #!/usr/bin/env bash
 set -Eeuo pipefail
 
-REPO="${NG_REPO:-a2899882/NexusGate}"
+REPO="${NG_REPO:-rogue/NexusGate}"
 BRANCH="${NG_BRANCH:-main}"
+# 自建 Gitea 分发基址（源码归档由此拼装）
+GIT_BASE="https://git.881218.xyz/${REPO}"
 DOMAIN=""
 EMAIL=""
 
@@ -61,7 +63,7 @@ fi
 node_major="$(node -p 'process.versions.node.split(`.`)[0]')"
 [[ "$node_major" -ge 18 ]] || die "需要 Node.js 18 或更高版本"
 
-# NG_LOCAL_SRC 指向本地源码目录时跳过 GitHub 下载（本地改动尚未 push 时的全新安装用这个）
+# NG_LOCAL_SRC 指向本地源码目录时跳过远程下载（本地改动尚未 push 时的全新安装用这个）
 src_dir=""
 if [[ -n "${NG_LOCAL_SRC:-}" ]]; then
   [[ -f "${NG_LOCAL_SRC}/server.js" ]] || die "NG_LOCAL_SRC 目录不完整（缺少 server.js）"
@@ -71,10 +73,10 @@ else
   tmp_dir="$(mktemp -d /tmp/nexusgate-install.XXXXXX)"
   trap 'rm -rf -- "$tmp_dir"' EXIT
   info "下载 ${REPO}@${BRANCH}"
-  curl -fL --retry 3 "https://github.com/${REPO}/archive/refs/heads/${BRANCH}.tar.gz" -o "$tmp_dir/source.tgz"
+  curl -fL --retry 3 "${GIT_BASE}/archive/${BRANCH}.tar.gz" -o "$tmp_dir/source.tgz"
   mkdir -p "$tmp_dir/source"
   tar -xzf "$tmp_dir/source.tgz" -C "$tmp_dir/source"
-  # GitHub tarball 解包后只有一层顶层目录；不用 --strip-components（BusyBox tar 不支持）
+  # 源码包解包后只有一层顶层目录；不用 --strip-components（BusyBox tar 不支持）
   src_dir="$(echo "$tmp_dir/source"/*/)"
   [[ -f "${src_dir}server.js" ]] || die "安装包不完整"
 fi