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.

public/app.js

@@ -535,7 +535,7 @@ document.addEventListener('click', async (event) => {
       toast('分配已移除'); const node = state.nodes.find((item) => item.id === itemId); await load('chains'); nodeClientsModal(state.nodes.find((item) => item.id === node.id));
     } else if (action === 'enroll-server') {
       const result = await api(`/api/servers/${itemId}/enrollment-token`, { method:'POST', body:'{}' });
-      const command = `curl -fsSL https://raw.githubusercontent.com/aaaamoooo/NexusGate/main/scripts/agent-install.sh | bash -s -- --server ${location.origin} --token ${result.token}`;
+      const command = `curl -fsSL https://git.881218.xyz/rogue/NexusGate/raw/branch/main/scripts/agent-install.sh | bash -s -- --server ${location.origin} --token ${result.token}`;
       modal('ONE-TIME ENROLLMENT', 'Agent 注册 / 重装命令', `<div class="stack"><div class="notice">令牌 30 分钟内有效且只能使用一次。重装会保留本机资源文件，并在 Agent 重启后自动与控制面对账。</div><div class="codebox">${esc(command)}</div><button class="primary" data-action="copy-uri" data-value="${esc(command)}">复制命令</button></div>`);
     } else if (action === 'copy-uri') { await copyText(button.dataset.value); toast('已复制到剪贴板'); }
     else if (action === 'qr-uri') {
@@ -546,7 +546,7 @@ document.addEventListener('click', async (event) => {
     }
     else if (action === 'agent-uninstall') {
       const command = 'ng-agent uninstall';
-      const fallback = 'curl -fsSL https://raw.githubusercontent.com/aaaamoooo/NexusGate/main/scripts/agent-uninstall.sh | bash';
+      const fallback = 'curl -fsSL https://git.881218.xyz/rogue/NexusGate/raw/branch/main/scripts/agent-uninstall.sh | bash';
       modal('AGENT MAINTENANCE', '在目标服务器 SSH 卸载 Agent', `<div class="stack"><div class="notice warning">先停用并删除关联节点，让在线 Agent 完成资源清理。卸载会停止 NexusGate 专属服务并移除密钥与节点配置。然后在控制台删除设备登记；离线且有待清理项时使用“遗忘离线设备”。遗忘不会远程卸载机器。</div><div class="codebox">${esc(command)}</div><button data-action="copy-uri" data-value="${esc(command)}">复制命令</button><small>旧版 Agent 没有 ng-agent 命令时，使用下列兼容命令：</small><div class="codebox">${esc(fallback)}</div><button data-action="copy-uri" data-value="${esc(fallback)}">复制兼容命令</button></div>`);
     }
     else if (action === 'retry-cleanup') { const result = await api(`/api/servers/${itemId}/cleanup`, { method:'POST', body:'{}' }); toast(result.queued ? `已重新排队 ${result.queued} 个清理任务` : '清理任务已在队列中，请等待设备上线'); await load(); }