feat(download): 新增历史版本选择，重构下载全链路逻辑与UI

- 重构lib/github-releases.ts：统一预览版判断逻辑，新增支持`vX.Y.Z-N.beta`格式的beta标签，提取复用函数简化代码，将原有重复工具方法抽取至此统一管理并新增平台资产匹配工具
- 新增通用PlatformDownloadGrid多平台下载卡片组件，标准化多平台下载UI实现
- 重构两个下载API接口：统一返回格式并新增历史版本列表数据
- 重构/download和/launcher/join-beta页面：替换使用新的下载组件，新增历史版本选择功能

components/platform-download-grid.tsx

@@ -0,0 +1,96 @@
+import React from "react";
+import {
+  Monitor,
+  Apple,
+  Terminal,
+  Download,
+  type LucideIcon,
+} from "lucide-react";
+import { Button } from "@/components/ui/button";
+import { cn } from "@/lib/utils";
+
+/**
+ * 平台下载网格（Windows / macOS / Linux 三卡）
+ * 三个平台按钮常驻；某平台没有对应安装包时按钮置灰禁用（不删除），
+ * 并在卡内提示"暂未提供此平台版本"。
+ */
+
+export interface PlatformDownload {
+  /** 展示文件名（可选） */
+  name?: string;
+  url: string;
+}
+
+export interface PlatformSlot {
+  key: string;
+  label: string;
+  icon: LucideIcon;
+  /** url 为空/缺失 = 该平台暂无版本，按钮禁用 */
+  download?: PlatformDownload | null;
+  /** 无版本时的提示文案 */
+  hint?: string;
+}
+
+interface PlatformDownloadGridProps {
+  platforms: PlatformSlot[];
+  onDownload: (slot: PlatformSlot) => void;
+  className?: string;
+}
+
+export function PlatformDownloadGrid({
+  platforms,
+  onDownload,
+  className,
+}: PlatformDownloadGridProps) {
+  return (
+    <div className={cn("grid grid-cols-1 sm:grid-cols-3 gap-4", className)}>
+      {platforms.map((p) => {
+        const available = !!p.download?.url;
+        return (
+          <div
+            key={p.key}
+            className={cn(
+              "rounded-2xl border p-6 flex flex-col gap-4 transition-all duration-300",
+              available
+                ? "border-border/50 bg-background/60 backdrop-blur-xl hover:scale-[1.02]"
+                : "border-border/30 opacity-60"
+            )}
+          >
+            <div className="flex items-center gap-3">
+              <div
+                className={cn(
+                  "flex items-center justify-center w-10 h-10 rounded-xl",
+                  available ? "bg-primary/10" : "bg-muted"
+                )}
+              >
+                <p.icon
+                  className={cn(
+                    "size-5",
+                    available ? "text-primary" : "text-muted-foreground"
+                  )}
+                />
+              </div>
+              <span className="font-semibold text-lg">{p.label}</span>
+            </div>
+            <div className="mt-auto">
+              <Button
+                className="w-full"
+                disabled={!available}
+                title={available ? undefined : "该平台暂未提供安装包"}
+                onClick={() => available && onDownload(p)}
+              >
+                <Download className="size-4" />
+                下载
+              </Button>
+              {!available && (
+                <p className="mt-2 text-center text-xs text-muted-foreground">
+                  {p.hint || "暂未提供此平台版本"}
+                </p>
+              )}
+            </div>
+          </div>
+        );
+      })}
+    </div>
+  );
+}