feat(client): add high-contrast theme option to theme switcher

Extend theme modes with an explicit high-contrast choice (WCAG 1.4.3) and expose i18n label keys for the theme menu.

client/src/composables/useThemeMode.ts

@@ -2,21 +2,27 @@ import { computed, watch } from 'vue'
 import { usePreferredDark, useStorage } from '@vueuse/core'
 import { useTheme } from 'vuetify'
 
-export type ThemeMode = 'light' | 'dark' | 'system'
-export type ResolvedTheme = 'light' | 'dark'
+export type ThemeMode = 'light' | 'dark' | 'highContrast' | 'system'
+export type ResolvedTheme = 'light' | 'dark' | 'highContrast'
 
 export interface ThemeOption {
   value: ThemeMode
-  title: string
+  labelKey: string
   icon: string
 }
 
 export const themeOptions: ThemeOption[] = [
-  { value: 'light', title: 'Jasny', icon: 'mdi-white-balance-sunny' },
-  { value: 'dark', title: 'Ciemny', icon: 'mdi-weather-night' },
-  { value: 'system', title: 'Systemowy', icon: 'mdi-theme-light-dark' },
+  { value: 'light', labelKey: 'theme.light', icon: 'mdi-white-balance-sunny' },
+  { value: 'dark', labelKey: 'theme.dark', icon: 'mdi-weather-night' },
+  { value: 'highContrast', labelKey: 'theme.highContrast', icon: 'mdi-contrast-circle' },
+  { value: 'system', labelKey: 'theme.system', icon: 'mdi-theme-light-dark' },
 ]
 
+/**
+ * Color-scheme handling: light / dark / high contrast / follow the system.
+ * `highContrast` is offered explicitly because the OS `system` preference only
+ * yields light or dark (WCAG 1.4.3 for low-vision users).
+ */
 export function useThemeMode() {
   const theme = useTheme()
   const preferredDark = usePreferredDark()