feat: 全局主题切换（浅色/深色/跟随系统）

- CSS 变量驱动的主题系统，所有颜色响应 data-theme 属性
- 新增语义化色彩 heading/secondary/tertiary，替换硬编码 text-white/text-gray-*
- 右上角三态主题按钮（自动/浅色/深色），全局可用无需登录
- layout.tsx 内联脚本防闪烁
- 修复个人中心页面溢出无法滚动

src/components/QrInviteModal.tsx

@@ -88,7 +88,7 @@ export default function QrInviteModal({
             </button>
 
             <div className="flex flex-col items-center">
-              <div className="flex items-center gap-2 text-white">
+              <div className="flex items-center gap-2 text-heading">
                 <QrCode size={18} className="text-accent" />
                 <h2 className="text-lg font-bold">邀请饭搭子</h2>
               </div>
@@ -116,7 +116,7 @@ export default function QrInviteModal({
               <div className="mt-5 flex w-full gap-2.5">
                 <button
                   onClick={handleCopy}
-                  className="flex h-11 flex-1 items-center justify-center gap-1.5 rounded-xl bg-elevated text-sm font-semibold text-gray-300 ring-1 ring-border transition-colors active:bg-subtle"
+                  className="flex h-11 flex-1 items-center justify-center gap-1.5 rounded-xl bg-elevated text-sm font-semibold text-secondary ring-1 ring-border transition-colors active:bg-subtle"
                 >
                   <Copy size={15} />
                   复制链接