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/agent-singbox.sh

@@ -14,7 +14,7 @@ if [[ -x "$target" && -x "$stats_target" && "${2:-}" != '--force' ]]; then
   exit 0
 fi
 
-# 优先从 GitHub Release 下载预编译二进制，免去现场编译
+# 优先从 Gitea Release 下载预编译二进制，免去现场编译
 # Release tag: sing-box-v1.13.21，需上传 nexusgate-sing-box-linux-<arch> 和 nexusgate-sing-box-stats-linux-<arch>
 try_prebuilt() {
   local arch dl_arch release_tag base_url tmp_bin tmp_stats
@@ -24,11 +24,11 @@ try_prebuilt() {
     *) return 1 ;;
   esac
   release_tag="${NG_SINGBOX_RELEASE:-sing-box-v1.13.21}"
-  base_url="https://github.com/aaaamoooo/NexusGate/releases/download/${release_tag}"
+  base_url="https://git.881218.xyz/${NG_REPO:-rogue/NexusGate}/releases/download/${release_tag}"
   tmp_bin="$(mktemp /tmp/nexusgate-singbox-bin.XXXXXX)"
   tmp_stats="$(mktemp /tmp/nexusgate-singbox-stats.XXXXXX)"
   trap 'rm -f -- "$tmp_bin" "$tmp_stats"' RETURN
-  printf '尝试从 GitHub Release 下载预编译 sing-box...\n'
+  printf '尝试从 Gitea Release 下载预编译 sing-box...\n'
   if curl -fL --retry 2 --connect-timeout 15 -o "$tmp_bin" "${base_url}/nexusgate-sing-box-linux-${dl_arch}" 2>/dev/null && \
      curl -fL --retry 2 --connect-timeout 15 -o "$tmp_stats" "${base_url}/nexusgate-sing-box-stats-linux-${dl_arch}" 2>/dev/null; then
     chmod +x "$tmp_bin" "$tmp_stats"
@@ -80,12 +80,12 @@ printf '构建 sing-box %s（启用 V2Ray 统计 API，首次需下载 Go 依赖
 GOBIN="$build_dir" GOTOOLCHAIN=auto GOMAXPROCS=1 go install -trimpath -ldflags='-s -w' -p 1 -tags with_v2ray_api "github.com/sagernet/sing-box/cmd/sing-box@${version}"
 [[ -s "$build_dir/sing-box" ]] || { printf 'sing-box 构建失败\n' >&2; exit 1; }
 
-repo="${NG_REPO:-a2899882/NexusGate}"
+repo="${NG_REPO:-rogue/NexusGate}"
 branch="${NG_BRANCH:-main}"
 if [[ -n "${NG_STATS_SOURCE:-}" ]]; then
   install -m 0600 "$NG_STATS_SOURCE" "$stats_source_dir/stats-query.go"
 else
-  curl -fL --retry 3 "https://raw.githubusercontent.com/${repo}/${branch}/agent/stats-query.go" -o "$stats_source_dir/stats-query.go"
+  curl -fL --retry 3 "https://git.881218.xyz/${repo}/raw/branch/${branch}/agent/stats-query.go" -o "$stats_source_dir/stats-query.go"
 fi
 ( cd "$stats_source_dir"
   GOTOOLCHAIN=auto go mod init nexusgate/statsquery