feat(frontend): add light/dark theme switcher

Add persisted theme mode (light, dark, system) with a header menu.
Rework the light and dark palettes for stronger contrast and drop the
high-contrast theme.

client/src/composables/useThemeMode.ts

@@ -0,0 +1,32 @@
+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 interface ThemeOption {
+  value: ThemeMode
+  title: 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' },
+]
+
+export function useThemeMode() {
+  const theme = useTheme()
+  const preferredDark = usePreferredDark()
+  const mode = useStorage<ThemeMode>('tarninahub-theme', 'system')
+
+  const resolved = computed<ResolvedTheme>(() =>
+    mode.value === 'system' ? (preferredDark.value ? 'dark' : 'light') : mode.value,
+  )
+
+  watch(resolved, (value) => theme.change(value), { immediate: true })
+
+  return { mode, resolved, preferredDark, themeOptions }
+}