feat(theme): 专业主题系统重构 — Warp/Linear 设计参考，深色/浅色完整适配

参考设计系统:
- Warp: 温暖暗色调、自然层次、克制配色
- Linear: 亮度阶梯层次、半透明边框、精确排版

核心改进:
- app-theme.ts: 完整重写深色/浅色调色板 (+surface-hover/active, border-subtle/focus, feedback-bg, placeholder)
- useTheme.ts: 修复硬编码 return 'dark'，支持 dark/light/system 完整切换
- index.css: 消除硬编码颜色，全部使用 CSS 变量
- tailwind.config.js: 新增 txt/content/surface/border/ph/feedback 语义 token
- 44 文件批量迁移: 292 处 text-gray/bg-gray/text-white/placeholder-gray → 主题感知语义 token

| 44 files +611/-482 | tsc零错误 | 96/96 tests passing

src/components/common/Toast.tsx

@@ -32,13 +32,13 @@ const colorMap: Record<ToastType, { bg: string; border: string; icon: string; te
     text: 'text-red-200',
   },
   warning: {
-    bg: 'bg-amber-900/40',
+    bg: 'bg-surface-basember-900/40',
     border: 'border-amber-700/60',
     icon: 'text-amber-400',
     text: 'text-amber-200',
   },
   info: {
-    bg: 'bg-blue-900/40',
+    bg: 'bg-surface-baselue-900/40',
     border: 'border-blue-700/60',
     icon: 'text-blue-400',
     text: 'text-blue-200',
@@ -109,7 +109,7 @@ function ToastItemView({ toast, onClose }: ToastItemProps) {
         className="p-0.5 rounded hover:bg-white/10 transition-colors shrink-0"
         aria-label="关闭通知"
       >
-        <X className="w-4 h-4 text-gray-400 hover:text-gray-200" />
+        <X className="w-4 h-4 text-txt-muted hover:text-content-primary" />
       </button>
     </div>
   )