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/components/ConfigPanel/index.tsx

@@ -1,17 +1,19 @@
 // ============================================================
 // ConfigPanel — 右侧抽屉式配置面板
-// 职责：TTS 开关、detail level 切换、语言选择
+// 职责：主题切换、TTS 开关、detail level 切换、语言选择
 // ============================================================
 
-import type { SessionConfig } from "../../types";
+import type { SessionConfig, Theme } from "../../types";
 
 interface ConfigPanelProps {
   config: SessionConfig;
+  theme: Theme;
   onUpdate: (partial: Partial<SessionConfig>) => void;
+  onThemeChange: (theme: Theme) => void;
   onClose: () => void;
 }
 
-export function ConfigPanel({ config, onUpdate, onClose }: ConfigPanelProps) {
+export function ConfigPanel({ config, theme, onUpdate, onThemeChange, onClose }: ConfigPanelProps) {
   return (
     <div className="drawer-overlay" onClick={onClose}>
       <div className="drawer" onClick={(e) => e.stopPropagation()}>
@@ -21,6 +23,24 @@ export function ConfigPanel({ config, onUpdate, onClose }: ConfigPanelProps) {
         </div>
 
         <div className="drawer__body">
+          <div className="config-group">
+            <div className="config-group__title">外观</div>
+
+            <label className="config-row">
+              <div className="config-row__info">
+                <span className="config-row__label">主题</span>
+                <span className="config-row__desc">切换明暗主题</span>
+              </div>
+              <select
+                value={theme}
+                onChange={(e) => onThemeChange(e.target.value as Theme)}
+              >
+                <option value="dark">深色</option>
+                <option value="light">浅色</option>
+              </select>
+            </label>
+          </div>
+
           <div className="config-group">
             <div className="config-group__title">会话</div>