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.

README.md

@@ -21,7 +21,7 @@
 
 ```bash
 # 克隆仓库
-git clone https://github.com/aaaamoooo/NexusGate.git /opt/nexusgate
+git clone https://git.881218.xyz/rogue/NexusGate.git /opt/nexusgate
 cd /opt/nexusgate
 
 # 安装依赖并启动
@@ -39,10 +39,10 @@ OpenWrt 上用 procd 常驻，见 `scripts/` 目录。
 3. 在节点机上以 root 运行该命令
 
 ```bash
-curl -fsSL https://raw.githubusercontent.com/aaaamoooo/NexusGate/main/scripts/agent-install.sh | bash -s -- --server https://主控域名 --token 你的TOKEN
+curl -fsSL https://git.881218.xyz/rogue/NexusGate/raw/branch/main/scripts/agent-install.sh | bash -s -- --server https://主控域名 --token 你的TOKEN
 ```
 
-Agent 是 Go 静态二进制，安装时从 GitHub Release 下载，节点机不再需要安装 Node.js。（如需回退旧版 Node Agent，安装时加环境变量 `NG_AGENT_IMPL=node`。）
+Agent 是 Go 静态二进制，安装时从 Gitea Release 下载，节点机不再需要安装 Node.js。（如需回退旧版 Node Agent，安装时加环境变量 `NG_AGENT_IMPL=node`。）
 
 ## 支持的协议