feat: add dark theme support with CSS custom properties and Naive UI integration

Implement runtime theme switching using CSS custom properties delegated through SCSS variables, with light/dark/system modes, FOUC prevention, sidebar toggle, and settings selector. Add theme-aware video assets for sidebar and chat thinking indicator.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/client/src/i18n/locales/en.ts

@@ -292,6 +292,11 @@ export default {
       bellOnCompleteHint: 'Play sound when AI finishes',
       busyInputMode: 'Busy Input Mode',
       busyInputModeHint: 'Allow input while AI is processing',
+      theme: 'Theme',
+      themeHint: 'Choose light, dark, or follow system preference',
+      themeLight: 'Light',
+      themeDark: 'Dark',
+      themeSystem: 'System',
     },
     agent: {
       maxTurns: 'Max Turns',

packages/client/src/i18n/locales/zh.ts

@@ -292,6 +292,11 @@ export default {
       bellOnCompleteHint: 'AI 回复完成时播放提示音',
       busyInputMode: '忙碌输入模式',
       busyInputModeHint: 'AI 处理中仍可输入',
+      theme: '主题',
+      themeHint: '选择浅色、暗色或跟随系统',
+      themeLight: '浅色',
+      themeDark: '暗色',
+      themeSystem: '跟随系统',
     },
     agent: {
       maxTurns: '最大轮次',