adopt CYBER neon-dark theme tokens

Replace the Pure Ink light/dark/comic theme system with the CYBER
single-theme dark palette from the reviewed prototype: OKLCH surfaces,
one neon-cyan accent, and monospace-first typography.

Arco components were reading colors from arco.css because no
ConfigProvider was mounted, so the theme values in arco-theme.ts never
took effect. Bridge CYBER into Arco through its --color-* variables in
global.scss instead, and align arco-theme.ts to match.

Brightness and style switches are gone: useTheme now pins dark, the
comic font faces and locale keys are removed, and ThemeSwitch.vue is
deleted along with its sidebar slot and display setting.

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

@@ -1575,11 +1575,6 @@ export default {
       notifyOnCompleteTestFailed: 'Failed to send test notification',
       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',
       chatInputHeight: 'Chat input height',
       chatInputHeightHint: 'Default chat input height in pixels on wide screens. Mobile keeps auto height.'
     },

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

@@ -1567,11 +1567,6 @@ export default {
       notifyOnCompleteTestFailed: '测试通知发送失败',
       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',
       chatInputHeight: '聊天输入框高度',
       chatInputHeightHint: '桌面端聊天输入框默认高度（像素）。移动端保持自动高度。'
     },