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.

tests/client/sidebar-search.test.ts

@@ -68,10 +68,6 @@ vi.mock('@/components/layout/LanguageSwitch.vue', () => ({
   default: { name: 'LanguageSwitch', template: '<div />' },
 }))
 
-vi.mock('@/components/layout/ThemeSwitch.vue', () => ({
-  default: { name: 'ThemeSwitch', template: '<div />' },
-}))
-
 vi.mock('@/components/common/RouteLinkItem.vue', () => ({
   default: {
     name: 'RouteLinkItem',
@@ -135,7 +131,6 @@ describe('AppSidebar navigation', () => {
           ProfileSelector: true,
           ModelSelector: true,
           LanguageSwitch: true,
-          ThemeSwitch: true,
         },
       },
     })
@@ -153,7 +148,6 @@ describe('AppSidebar navigation', () => {
           ProfileSelector: true,
           ModelSelector: true,
           LanguageSwitch: true,
-          ThemeSwitch: true,
         },
       },
     })
@@ -181,7 +175,6 @@ describe('AppSidebar navigation', () => {
           ProfileSelector: true,
           ModelSelector: true,
           LanguageSwitch: true,
-          ThemeSwitch: true,
         },
       },
     })
@@ -200,7 +193,6 @@ describe('AppSidebar navigation', () => {
           ProfileSelector: true,
           ModelSelector: true,
           LanguageSwitch: true,
-          ThemeSwitch: true,
         },
       },
     })
@@ -225,7 +217,6 @@ describe('AppSidebar navigation', () => {
           ProfileSelector: true,
           ModelSelector: true,
           LanguageSwitch: true,
-          ThemeSwitch: true,
         },
       },
     })