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/RoomManageModal.tsx

@@ -118,7 +118,7 @@ export default function RoomManageModal({
 
             <div className="flex items-center gap-2">
               <Crown size={18} className="text-amber-400" />
-              <h2 className="text-lg font-bold text-white">房间管理</h2>
+              <h2 className="text-lg font-bold text-heading">房间管理</h2>
             </div>
             <p className="mt-1 text-xs text-muted">
               房间号 {roomId}
@@ -132,7 +132,7 @@ export default function RoomManageModal({
                 className={`flex h-11 w-full items-center justify-center gap-2 rounded-xl text-sm font-semibold transition-colors disabled:opacity-50 ${
                   locked
                     ? "bg-accent/15 text-accent ring-1 ring-accent/30 active:bg-accent/25"
-                    : "bg-elevated text-gray-300 ring-1 ring-border active:bg-subtle"
+                    : "bg-elevated text-secondary ring-1 ring-border active:bg-subtle"
                 }`}
               >
                 {loading === "lock" || loading === "unlock" ? (
@@ -179,7 +179,7 @@ export default function RoomManageModal({
                               房主
                             </span>
                           )}
-                          <span className="truncate text-xs font-medium text-gray-400">
+                          <span className="truncate text-xs font-medium text-tertiary">
                             {displayName}
                           </span>
                         </div>
@@ -211,7 +211,7 @@ export default function RoomManageModal({
                               </button>
                               <button
                                 onClick={() => setConfirmKick(null)}
-                                className="rounded-lg bg-subtle px-2.5 py-1 text-[11px] font-semibold text-gray-400 transition-colors active:bg-elevated"
+                                className="rounded-lg bg-subtle px-2.5 py-1 text-[11px] font-semibold text-tertiary transition-colors active:bg-elevated"
                               >
                                 取消
                               </button>
@@ -255,7 +255,7 @@ export default function RoomManageModal({
                     </button>
                     <button
                       onClick={() => setConfirmEnd(false)}
-                      className="flex h-9 flex-1 items-center justify-center rounded-lg bg-elevated text-xs font-semibold text-gray-400 transition-colors active:bg-subtle"
+                      className="flex h-9 flex-1 items-center justify-center rounded-lg bg-elevated text-xs font-semibold text-tertiary transition-colors active:bg-subtle"
                     >
                       再等等
                     </button>