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/DefaultFallback.tsx

@@ -18,13 +18,13 @@ export default function DefaultFallback({ error, onReset }: DefaultFallbackProps
             </svg>
           </div>
           <div>
-            <h2 className="text-lg font-semibold text-gray-200">出错了</h2>
-            <p className="text-sm text-gray-500">应用遇到意外问题</p>
+            <h2 className="text-lg font-semibold text-content-primary">出错了</h2>
+            <p className="text-sm text-txt-muted">应用遇到意外问题</p>
           </div>
         </div>
 
         <div className="bg-dark-900 rounded-lg p-4 mb-4">
-          <p className="text-sm text-gray-300 mb-2">{appError.message}</p>
+          <p className="text-sm text-content-secondary mb-2">{appError.message}</p>
           {appError.recoverable && (
             <p className="text-xs text-green-400">此错误可以恢复，请尝试刷新页面</p>
           )}
@@ -33,19 +33,19 @@ export default function DefaultFallback({ error, onReset }: DefaultFallbackProps
         <div className="flex gap-3">
           <button
             onClick={onReset}
-            className="flex-1 px-4 py-2 bg-accent-blue hover:bg-blue-600 text-white rounded-lg transition-colors text-sm font-medium"
+            className="flex-1 px-4 py-2 bg-surface-baseccent-blue hover:bg-surface-baselue-600 text-content-primary rounded-lg transition-colors text-sm font-medium"
           >
             刷新页面
           </button>
           <button
             onClick={() => window.location.reload()}
-            className="px-4 py-2 bg-dark-700 hover:bg-dark-600 text-gray-300 rounded-lg transition-colors text-sm"
+            className="px-4 py-2 bg-dark-700 hover:bg-dark-600 text-content-secondary rounded-lg transition-colors text-sm"
           >
             重启应用
           </button>
         </div>
 
-        <p className="mt-4 text-xs text-gray-600 text-center">
+        <p className="mt-4 text-xs text-txt-faint text-center">
           如果问题持续存在，请检查日志或联系支持
         </p>
       </div>