feat: 下载中心对接 GitHub Releases 并适配 latest.yml

- 新增 /download 下载页：正式版/预览版双渠道、latest.yml/latest-beta.yml 数据源、镜像下载源切换、SHA512 校验展示
- 新增 /api/download-version：GitHub API 自动识别 + 镜像兜底 + yml 解析 + 旧源兜底
- /launcher/join-beta 改为自动识别 GitHub 预览版，支持镜像下载源切换
- 新增 markdown-lite 轻量渲染器、lib/download-sources 与 lib/github-releases 共享模块
- 头部导航新增下载中心入口

components/markdown-lite.tsx

@@ -0,0 +1,176 @@
+import React from "react";
+import { cn } from "@/lib/utils";
+
+/**
+ * 轻量 Markdown 渲染器（无依赖）
+ * 用于渲染 GitHub Release Notes 这类常见格式：
+ * 标题、无序/有序列表、引用、代码块、粗体、行内代码、链接、分隔线、段落
+ */
+
+function renderInline(text: string, keyPrefix: string): React.ReactNode[] {
+  const parts: React.ReactNode[] = [];
+  const regex = /(\*\*[^*]+\*\*|`[^`]+`|\[[^\]]+\]\([^)]+\))/g;
+  let last = 0;
+  let m: RegExpExecArray | null;
+  let i = 0;
+
+  while ((m = regex.exec(text)) !== null) {
+    if (m.index > last) {
+      parts.push(text.slice(last, m.index));
+    }
+    const tok = m[0];
+    if (tok.startsWith("**")) {
+      parts.push(
+        <strong key={`${keyPrefix}-b${i}`} className="font-semibold">
+          {tok.slice(2, -2)}
+        </strong>
+      );
+    } else if (tok.startsWith("`")) {
+      parts.push(
+        <code
+          key={`${keyPrefix}-c${i}`}
+          className="rounded bg-muted px-1 py-0.5 font-mono text-[0.85em]"
+        >
+          {tok.slice(1, -1)}
+        </code>
+      );
+    } else {
+      const link = tok.match(/^\[([^\]]+)\]\(([^)]+)\)$/);
+      if (link) {
+        parts.push(
+          <a
+            key={`${keyPrefix}-l${i}`}
+            href={link[2]}
+            target="_blank"
+            rel="noopener noreferrer"
+            className="text-primary underline underline-offset-3 hover:text-primary/80"
+          >
+            {link[1]}
+          </a>
+        );
+      } else {
+        parts.push(tok);
+      }
+    }
+    last = m.index + tok.length;
+    i += 1;
+  }
+  if (last < text.length) {
+    parts.push(text.slice(last));
+  }
+  return parts;
+}
+
+export function MarkdownLite({
+  text,
+  className,
+}: {
+  text: string;
+  className?: string;
+}) {
+  const lines = text.split(/\r?\n/);
+  const blocks: React.ReactNode[] = [];
+  let key = 0;
+
+  let inCode = false;
+  let codeBuf: string[] = [];
+
+  const push = (node: React.ReactNode) => {
+    blocks.push(<div key={key++}>{node}</div>);
+  };
+
+  for (const line of lines) {
+    // 围栏代码块
+    if (/^```/.test(line.trim())) {
+      if (inCode) {
+        push(
+          <pre className="overflow-x-auto rounded-lg border border-border/50 bg-muted/40 p-3 font-mono text-xs leading-relaxed">
+            {codeBuf.join("\n")}
+          </pre>
+        );
+        codeBuf = [];
+        inCode = false;
+      } else {
+        inCode = true;
+      }
+      continue;
+    }
+    if (inCode) {
+      codeBuf.push(line);
+      continue;
+    }
+
+    const trimmed = line.trim();
+    if (!trimmed) continue; // 空行跳过
+
+    // 标题
+    const heading = trimmed.match(/^(#{1,4})\s+(.*)$/);
+    if (heading) {
+      const lv = heading[1].length;
+      const content = renderInline(heading[2], `h${key}`);
+      const cls = {
+        1: "text-xl font-bold tracking-tight",
+        2: "text-lg font-bold tracking-tight",
+        3: "text-base font-semibold",
+        4: "text-sm font-semibold",
+      }[lv as 1 | 2 | 3 | 4];
+      push(<h3 className={cn("mb-1 mt-2", cls)}>{content}</h3>);
+      continue;
+    }
+
+    // 分隔线
+    if (/^-{3,}$/.test(trimmed)) {
+      push(<hr className="my-3 border-border/60" />);
+      continue;
+    }
+
+    // 引用
+    if (/^>/.test(trimmed)) {
+      push(
+        <blockquote className="my-1 border-l-2 border-primary/50 pl-3 text-muted-foreground">
+          {renderInline(trimmed.replace(/^>\s?/, ""), `q${key}`)}
+        </blockquote>
+      );
+      continue;
+    }
+
+    // 无序列表
+    const ul = trimmed.match(/^[-*]\s+(.*)$/);
+    if (ul) {
+      push(
+        <div className="flex items-start gap-2 py-0.5 text-sm">
+          <span className="mt-1.5 size-1.5 shrink-0 rounded-full bg-primary" />
+          <span className="text-muted-foreground">
+            {renderInline(ul[1], `ul${key}`)}
+          </span>
+        </div>
+      );
+      continue;
+    }
+
+    // 有序列表
+    const ol = trimmed.match(/^\d+\.\s+(.*)$/);
+    if (ol) {
+      push(
+        <div className="flex items-start gap-2 py-0.5 text-sm">
+          <span className="shrink-0 font-mono text-xs leading-5 text-primary/80">
+            {trimmed.match(/^\d+/)?.[0]}.
+          </span>
+          <span className="text-muted-foreground">
+            {renderInline(ol[1], `ol${key}`)}
+          </span>
+        </div>
+      );
+      continue;
+    }
+
+    // 普通段落
+    push(
+      <p className="py-0.5 text-sm leading-relaxed text-muted-foreground">
+        {renderInline(trimmed, `p${key}`)}
+      </p>
+    );
+  }
+
+  return <div className={cn("flex flex-col", className)}>{blocks}</div>;
+}