feat: add server-synced user preferences infrastructure (#484)

- introduce the foundational layer for persisting user preferences
to the server
- add UserPreferencesSchema and shared types for user preferences
- add client-only sync composable with debounced saves, route guard flush, and sendBeacon fallback
- integrate server sync into useSettings and migrate to shared UserPreferences type
- extract generic localStorage helpers, migrate consumers, remove usePreferencesProvider

Author: gusa4grr

app/composables/useLocalStorageHashProvider.ts

@@ -0,0 +1,42 @@
+import { createDefu } from 'defu'
+import { createLocalStorageProvider } from '~/utils/storage'
+
+const defu = createDefu((object, key, value) => {
+  if (Array.isArray(object[key]) && Array.isArray(value)) {
+    object[key] = value
+    return true
+  }
+})
+
+export function useLocalStorageHashProvider<T extends object>(key: string, defaultValue: T) {
+  const provider = createLocalStorageProvider<T>(key)
+  const data = ref<T>(defaultValue)
+
+  onMounted(() => {
+    const stored = provider.get()
+    if (stored) {
+      data.value = defu(stored, defaultValue)
+    }
+  })
+
+  function save() {
+    provider.set(data.value)
+  }
+
+  function reset() {
+    data.value = { ...defaultValue }
+    provider.remove()
+  }
+
+  function update<K extends keyof T>(key: K, value: T[K]) {
+    data.value[key] = value
+    save()
+  }
+
+  return {
+    data,
+    save,
+    reset,
+    update,
+  }
+}

app/composables/usePackageListPreferences.ts

@@ -11,18 +11,18 @@ import type {
 } from '#shared/types/preferences'
 import { DEFAULT_COLUMNS, DEFAULT_PREFERENCES } from '#shared/types/preferences'
 
+const STORAGE_KEY = 'npmx-list-prefs'
+
 /**
  * Composable for managing package list display preferences
  * Persists to localStorage and provides reactive state
- *
  */
 export function usePackageListPreferences() {
   const {
     data: preferences,
-    isHydrated,
     save,
     reset,
-  } = usePreferencesProvider<PackageListPreferences>(DEFAULT_PREFERENCES)
+  } = useLocalStorageHashProvider<PackageListPreferences>(STORAGE_KEY, DEFAULT_PREFERENCES)
 
   // Computed accessors for common properties
   const viewMode = computed({
@@ -90,7 +90,6 @@ export function usePackageListPreferences() {
   return {
     // Raw preferences
     preferences,
-    isHydrated,
 
     // Individual properties with setters
     viewMode,