feat(help): 新增应用内帮助页 /help（功能介绍 + 新手上手）

- 暖深色英雄 + 快速上手三步 + 三个核心理念 + 功能一览(写作/资料库/审阅/工具全局)
  + 顺手小技巧 + 常见问题，全部用设计系统组件与语义 token
- 内容对齐 PRODUCT_SPEC：立项→设定→大纲→写章→四审→验收、四审四维、多提供商网关、单一真相源
- 顶栏新增「帮助」入口（FileQuestion → /help）

apps/web/components/AppShell.tsx

@@ -1,6 +1,6 @@
 import Link from "next/link";
 import type { ReactNode } from "react";
-import { Settings } from "lucide-react";
+import { FileQuestion, Settings } from "lucide-react";
 
 import type { ActiveNav } from "@/lib/nav/items";
 import { buttonClass, focusRing } from "@/lib/ui/variants";
@@ -68,6 +68,19 @@ export function AppShell({
         >
           <CommandSearchButton />
           <ThemeToggle />
+          <Link
+            href="/help"
+            aria-label="帮助"
+            title="帮助"
+            className={buttonClass({
+              variant: "ghost",
+              size: "sm",
+              className: "border-transparent",
+            })}
+          >
+            <FileQuestion className="h-4 w-4" aria-hidden="true" />
+            <span className="hidden sm:inline">帮助</span>
+          </Link>
           <Link
             href="/settings/providers"
             aria-label="设置"