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/profile/page.tsx

@@ -266,7 +266,7 @@ export default function ProfilePage() {
       : `https://www.google.com/maps/search/?api=1&query=${encodeURIComponent(r.name)}`;
 
   return (
-    <div className="min-h-dvh bg-background pb-16 overflow-y-auto scrollbar-none">
+    <div className="h-dvh bg-background pb-16 overflow-y-auto scrollbar-none">
       <nav className="sticky top-0 z-10 flex h-14 items-center gap-3 bg-background/80 px-4 backdrop-blur-sm">
         <button
           onClick={() => router.push("/")}
@@ -324,10 +324,10 @@ export default function ProfilePage() {
                 </div>
               ) : (
                 <div className="flex items-center gap-2">
-                  <h2 className="text-lg font-bold text-white">{profile.username}</h2>
+                  <h2 className="text-lg font-bold text-heading">{profile.username}</h2>
                   <button
                     onClick={() => { setEditingUsername(true); setNewUsername(profile.username); }}
-                    className="text-muted transition-colors active:text-gray-300"
+                    className="text-muted transition-colors active:text-secondary"
                   >
                     <Edit3 size={13} />
                   </button>
@@ -385,7 +385,7 @@ export default function ProfilePage() {
             className="flex w-full items-center gap-2"
           >
             <Lock size={15} className="text-muted" />
-            <h3 className="text-sm font-semibold text-gray-300">修改密码</h3>
+            <h3 className="text-sm font-semibold text-secondary">修改密码</h3>
           </button>
 
           <AnimatePresence>
@@ -405,7 +405,7 @@ export default function ProfilePage() {
                         type={showPassword ? "text" : "password"}
                         value={currentPassword}
                         onChange={(e) => { setCurrentPassword(e.target.value); setPasswordMsg(""); }}
-                        className="h-9 w-full rounded-lg border-none bg-elevated px-3 pr-9 text-sm text-white outline-none ring-1 ring-border focus:ring-2 focus:ring-accent/50"
+                        className="h-9 w-full rounded-lg border-none bg-elevated px-3 pr-9 text-sm text-heading outline-none ring-1 ring-border focus:ring-2 focus:ring-accent/50"
                       />
                       <button
                         type="button"
@@ -423,7 +423,7 @@ export default function ProfilePage() {
                       value={newPassword}
                       onChange={(e) => { setNewPassword(e.target.value); setPasswordMsg(""); }}
                       placeholder="至少 6 个字符"
-                      className="mt-1 h-9 w-full rounded-lg border-none bg-elevated px-3 text-sm text-white outline-none ring-1 ring-border placeholder:text-dim focus:ring-2 focus:ring-accent/50"
+                      className="mt-1 h-9 w-full rounded-lg border-none bg-elevated px-3 text-sm text-heading outline-none ring-1 ring-border placeholder:text-dim focus:ring-2 focus:ring-accent/50"
                     />
                   </div>
                   <div>
@@ -433,7 +433,7 @@ export default function ProfilePage() {
                       value={confirmPassword}
                       onChange={(e) => { setConfirmPassword(e.target.value); setPasswordMsg(""); }}
                       placeholder="再次输入新密码"
-                      className="mt-1 h-9 w-full rounded-lg border-none bg-elevated px-3 text-sm text-white outline-none ring-1 ring-border placeholder:text-dim focus:ring-2 focus:ring-accent/50"
+                      className="mt-1 h-9 w-full rounded-lg border-none bg-elevated px-3 text-sm text-heading outline-none ring-1 ring-border placeholder:text-dim focus:ring-2 focus:ring-accent/50"
                     />
                   </div>
 
@@ -465,7 +465,7 @@ export default function ProfilePage() {
         >
           <div className="flex items-center gap-2">
             <Mail size={15} className="text-muted" />
-            <h3 className="text-sm font-semibold text-gray-300">绑定邮箱</h3>
+            <h3 className="text-sm font-semibold text-secondary">绑定邮箱</h3>
             <span className="text-[10px] text-dim">（可选）</span>
           </div>
           <div className="mt-3 flex gap-2">
@@ -477,7 +477,7 @@ export default function ProfilePage() {
                 setEmail(e.target.value);
                 setEmailMsg("");
               }}
-              className="h-9 flex-1 rounded-lg border-none bg-elevated px-3 text-sm text-white outline-none ring-1 ring-border placeholder:text-dim focus:ring-2 focus:ring-accent/50"
+              className="h-9 flex-1 rounded-lg border-none bg-elevated px-3 text-sm text-heading outline-none ring-1 ring-border placeholder:text-dim focus:ring-2 focus:ring-accent/50"
             />
             <button
               onClick={handleSaveEmail}
@@ -507,7 +507,7 @@ export default function ProfilePage() {
           >
             <div className="flex items-center gap-2">
               <Clock size={15} className="text-muted" />
-              <h3 className="text-sm font-semibold text-gray-300">
+              <h3 className="text-sm font-semibold text-secondary">
                 决策记录 {history.length > 0 && `(${history.length})`}
               </h3>
             </div>
@@ -556,7 +556,7 @@ export default function ProfilePage() {
                           />
                         )}
                         <div className="flex min-w-0 flex-1 flex-col justify-center">
-                          <p className="truncate text-sm font-semibold text-white">{d.restaurantName}</p>
+                          <p className="truncate text-sm font-semibold text-heading">{d.restaurantName}</p>
                           <div className="mt-0.5 flex items-center gap-2 text-[11px] text-muted">
                             <span>{d.matchType === "unanimous" ? "全员一致" : "最佳匹配"}</span>
                             <span>{d.participants} 人参与</span>
@@ -585,7 +585,7 @@ export default function ProfilePage() {
           >
             <div className="flex items-center gap-2">
               <Star size={15} className="text-muted" />
-              <h3 className="text-sm font-semibold text-gray-300">
+              <h3 className="text-sm font-semibold text-secondary">
                 收藏餐厅 {favorites.length > 0 && `(${favorites.length})`}
               </h3>
             </div>
@@ -633,7 +633,7 @@ export default function ProfilePage() {
                             />
                           )}
                           <div className="flex min-w-0 flex-1 flex-col justify-center">
-                            <p className="truncate text-sm font-semibold text-white">{r.name}</p>
+                            <p className="truncate text-sm font-semibold text-heading">{r.name}</p>
                             <div className="mt-0.5 flex items-center gap-2 text-[11px] text-muted">
                               <span className="flex items-center gap-0.5">
                                 <Star size={10} className="fill-amber-400 text-amber-400" />
@@ -684,7 +684,7 @@ export default function ProfilePage() {
       <AnimatePresence>
         {toast && (
           <motion.div
-            className="fixed left-1/2 top-10 z-60 -translate-x-1/2 rounded-xl bg-elevated px-4 py-2.5 text-xs font-medium text-white shadow-lg ring-1 ring-subtle"
+            className="fixed left-1/2 top-10 z-60 -translate-x-1/2 rounded-xl bg-elevated px-4 py-2.5 text-xs font-medium text-heading shadow-lg ring-1 ring-subtle"
             initial={{ opacity: 0, y: -12, x: "-50%" }}
             animate={{ opacity: 1, y: 0, x: "-50%" }}
             exit={{ opacity: 0, y: -12, x: "-50%" }}