refactor: unify user settings (#821)

Co-authored-by: 三咲智子 <sxzz@sxzz.moe>

composables/settings/index.ts

@@ -0,0 +1,21 @@
+import type { FeatureFlags } from './featureFlags'
+import type { ColorMode, FontSize } from '~/types'
+import { STORAGE_KEY_SETTINGS } from '~/constants'
+
+export interface UserSettings {
+  featureFlags: Partial<FeatureFlags>
+  colorMode?: ColorMode
+  fontSize?: FontSize
+  lang?: string
+  zenMode?: boolean
+}
+
+export function getDefaultUserSettings(): UserSettings {
+  return {
+    featureFlags: {},
+  }
+}
+
+export const userSettings = process.server
+  ? computed(getDefaultUserSettings)
+  : useUserLocalStorage(STORAGE_KEY_SETTINGS, getDefaultUserSettings)