refactor(backend): 启动环境初始化与资源项目化（零跨项目依赖）

- main.py：uv run main.py 一键启动——读取 backend/.env、TORCH_HOME/NLTK_DATA 指向项目内缓存、certifi SSL + NO_PROXY 网络直连、默认开启 s2s，启动横幅
- vendor/：补丁后 s2s 云化栈移入项目（backend/vendor/s2s-cloud），pyproject sources 改相对路径
- .gitignore：忽略 .torch-cache / nltk_data 缓存
- AGENTS.md：进程管理铁律（AI 不启动/重启服务，由用户操作）

backend/vendor/s2s-cloud/demo/ui/dom.js

@@ -0,0 +1,37 @@
+// @ts-check
+/** Small shared helpers used across the UI modules: a strict query selector,
+ *  HTML escaping for text we drop into innerHTML, error-string trimming, and
+ *  the opt-in debug flag. */
+
+/** Opt-in tracing: `localStorage.setItem("s2s.debug", "1")` then reload. */
+export const DEBUG = (() => {
+  try {
+    return localStorage.getItem("s2s.debug") === "1";
+  } catch {
+    return false;
+  }
+})();
+
+/**
+ * Query a single element, throwing if it's missing (so a broken selector fails
+ * loudly at startup rather than as a later null-deref).
+ * @template {HTMLElement} T
+ * @param {string} selector
+ * @returns {T}
+ */
+export function $(selector) {
+  const el = document.querySelector(selector);
+  if (!el) throw new Error(`Missing element: ${selector}`);
+  return /** @type {T} */ (el);
+}
+
+/** @param {string} s @returns {string} */
+export function escHtml(s) {
+  return s.replace(/&/g, "&amp;").replace(/</g, "&lt;").replace(/>/g, "&gt;");
+}
+
+/** Trim a long error message to fit the orb caption. @param {string} text */
+export function truncateError(text) {
+  if (text.length <= 90) return text;
+  return text.slice(0, 87) + "…";
+}