feat: 添加日间/夜间主题切换系统

- 新增 theme.js 主题管理模块（localStorage 持久化）
- variables.css 重构为亮色默认 + 暗色 data-theme 切换
- sidebar 底部添加主题切换按钮（sun/moon SVG 图标）
- 修复 scrollbar 硬编码颜色为 CSS 变量
- 修复 agents.js fallbacks 未定义时的空指针错误

src/components/sidebar.js

@@ -2,6 +2,7 @@
  * 侧边导航栏
  */
 import { navigate, getCurrentRoute } from '../router.js'
+import { toggleTheme, getTheme } from '../lib/theme.js'
 
 const NAV_ITEMS = [
   {
@@ -68,10 +69,31 @@ export function renderSidebar(el) {
   }
 
   html += '</nav>'
+
+  // 主题切换按钮
+  const isDark = getTheme() === 'dark'
+  const sunIcon = '<svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2"><circle cx="12" cy="12" r="5"/><line x1="12" y1="1" x2="12" y2="3"/><line x1="12" y1="21" x2="12" y2="23"/><line x1="4.22" y1="4.22" x2="5.64" y2="5.64"/><line x1="18.36" y1="18.36" x2="19.78" y2="19.78"/><line x1="1" y1="12" x2="3" y2="12"/><line x1="21" y1="12" x2="23" y2="12"/><line x1="4.22" y1="19.78" x2="5.64" y2="18.36"/><line x1="18.36" y1="5.64" x2="19.78" y2="4.22"/></svg>'
+  const moonIcon = '<svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2"><path d="M21 12.79A9 9 0 1111.21 3 7 7 0 0021 12.79z"/></svg>'
+
+  html += `
+    <div class="sidebar-footer">
+      <div class="nav-item" id="btn-theme-toggle">
+        ${isDark ? sunIcon : moonIcon}
+        <span>${isDark ? '日间模式' : '夜间模式'}</span>
+      </div>
+    </div>
+  `
+
   el.innerHTML = html
 
-  // 绑定点击事件
-  el.querySelectorAll('.nav-item').forEach(item => {
+  // 绑定导航点击事件
+  el.querySelectorAll('.nav-item[data-route]').forEach(item => {
     item.onclick = () => navigate(item.dataset.route)
   })
+
+  // 主题切换
+  el.querySelector('#btn-theme-toggle')?.addEventListener('click', () => {
+    toggleTheme()
+    renderSidebar(el)
+  })
 }