feat: 增加深色/浅色主题切换功能

- types 新增 Theme 类型（dark/light）
- storage 新增 loadTheme/saveTheme 持久化主题偏好
- ConfigPanel 新增外观分组，主题下拉选择
- App.tsx 通过 data-theme 属性挂载到 html 元素
- App.css 新增 [data-theme=light] 亮色变量覆盖

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/src/lib/storage.ts

@@ -3,9 +3,10 @@
 // 职责：会话配置持久化
 // ============================================================
 
-import type { SessionConfig } from "../types";
+import type { SessionConfig, Theme } from "../types";
 
 const CONFIG_KEY = "camtalk:config";
+const THEME_KEY = "camtalk:theme";
 
 const DEFAULT_CONFIG: SessionConfig = {
   ttsEnabled: true,
@@ -33,3 +34,19 @@ export function saveConfig(config: SessionConfig): void {
     // localStorage 不可用时静默失败
   }
 }
+
+/** 加载主题偏好 */
+export function loadTheme(): Theme {
+  try {
+    const raw = localStorage.getItem(THEME_KEY);
+    if (raw === "light" || raw === "dark") return raw;
+  } catch { /* ignore */ }
+  return "dark";
+}
+
+/** 保存主题偏好 */
+export function saveTheme(theme: Theme): void {
+  try {
+    localStorage.setItem(THEME_KEY, theme);
+  } catch { /* ignore */ }
+}