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/app/invite/[id]/page.tsx

@@ -73,7 +73,7 @@ export default function InvitePage() {
         <div className="flex h-16 w-16 items-center justify-center rounded-2xl bg-surface ring-1 ring-border">
           <Utensils size={28} className="text-muted" />
         </div>
-        <h1 className="text-xl font-bold text-white">房间不存在</h1>
+        <h1 className="text-xl font-bold text-heading">房间不存在</h1>
         <p className="text-center text-sm text-muted">
           这个房间已过期或不存在，请让朋友重新分享链接
         </p>
@@ -99,7 +99,7 @@ export default function InvitePage() {
           {scene === "drink" ? <Coffee size={28} className="text-white" /> : <Utensils size={28} className="text-white" />}
         </div>
 
-        <h1 className="mt-5 text-3xl font-black tracking-tight text-white">
+        <h1 className="mt-5 text-3xl font-black tracking-tight text-heading">
           NoWhatever
         </h1>
         <p className="mt-0.5 text-sm font-medium tracking-widest text-muted">
@@ -113,7 +113,7 @@ export default function InvitePage() {
         animate={{ y: 0, opacity: 1 }}
         transition={{ duration: 0.5, delay: 0.1 }}
       >
-        <p className="text-center text-lg font-bold text-white">
+        <p className="text-center text-lg font-bold text-heading">
           {sceneConfig.inviteText}
         </p>
         <div className="flex items-center gap-2 text-sm text-muted">
@@ -141,7 +141,7 @@ export default function InvitePage() {
           <div className="flex h-10 w-10 items-center justify-center rounded-full bg-accent/15">
             <Users size={18} className="text-accent" />
           </div>
-          <span className="text-xs font-semibold text-gray-300">加入房间</span>
+          <span className="text-xs font-semibold text-secondary">加入房间</span>
           <span className="text-[10px] leading-tight text-muted text-center">
             和朋友一起
           </span>
@@ -153,7 +153,7 @@ export default function InvitePage() {
           <div className="flex h-10 w-10 items-center justify-center rounded-full bg-amber-500/15">
             <Heart size={18} className="text-amber-400" />
           </div>
-          <span className="text-xs font-semibold text-gray-300">各自滑卡</span>
+          <span className="text-xs font-semibold text-secondary">各自滑卡</span>
           <span className="text-[10px] leading-tight text-muted text-center">
             右滑喜欢的店
           </span>
@@ -165,7 +165,7 @@ export default function InvitePage() {
           <div className="flex h-10 w-10 items-center justify-center rounded-full bg-rose-500/15">
             <Sparkles size={18} className="text-rose-400" />
           </div>
-          <span className="text-xs font-semibold text-gray-300">匹配结果</span>
+          <span className="text-xs font-semibold text-secondary">匹配结果</span>
           <span className="text-[10px] leading-tight text-muted text-center">
             滑中同一家就去
           </span>