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/components/layout/AppSidebar.vue

@@ -9,7 +9,6 @@ import RouteLinkItem from '@/components/common/RouteLinkItem.vue'
 import ModelSelector from "@/components/layout/ModelSelector.vue";
 import ProfileSelector from "@/components/layout/ProfileSelector.vue";
 import LanguageSwitch from "@/components/layout/LanguageSwitch.vue";
-import ThemeSwitch from "@/components/layout/ThemeSwitch.vue";
 import { changelog } from "@/data/changelog";
 import { getStoredUsername, isStoredSuperAdmin } from "@/api/client";
 
@@ -350,7 +349,6 @@ function handleUpdateClick() {
         >
           Studio v{{ appStore.serverVersion || "0.1.0" }}
         </span>
-        <ThemeSwitch />
       </div>
       <a-button v-if="appStore.clientOutdated" type="primary" status="warning" size="mini" long class="update-btn" @click="handleReloadClient">
         {{ t('sidebar.reloadClientVersion', { version: appStore.serverVersion }) }}
@@ -662,10 +660,6 @@ function handleUpdateClick() {
   }
 }
 
-.version-info :deep(.theme-switch-container) {
-  flex-shrink: 0;
-}
-
 .update-btn {
   margin: 4px 0 0;
   border-radius: $radius-sm;