fix(appearance): Theme-Einstellungen serverseitig speichern und beim Login wiederherstellen

Nach PWA-Cache-Löschung gingen Theme und Farbschema verloren, weil sie nur in localStorage lagen. Die Präferenzen werden jetzt synchronisiert und nach dem Login erneut angewendet.

Co-authored-by: Cursor <cursoragent@cursor.com>

client/src/services/appearancePrefs.ts

@@ -0,0 +1,62 @@
+import { apiJson } from './api.js'
+import { notifyAppearanceChanged } from './appearance.js'
+import {
+  getActiveUserId,
+  getColorSchemePreference,
+  getThemePreference,
+  setColorSchemePreference,
+  setThemePreference
+} from './userPreferences.js'
+
+const API_BASE = '/api/auth/appearance-prefs'
+
+export interface AppearancePrefs {
+  theme: string
+  colorScheme: string
+  persisted: boolean
+}
+
+function hasLocalAppearancePrefs(userId: string): boolean {
+  return (
+    localStorage.getItem(`user_pref_theme_${userId}`) != null ||
+    localStorage.getItem(`user_pref_color_scheme_${userId}`) != null
+  )
+}
+
+export async function fetchAppearancePrefs(): Promise<AppearancePrefs> {
+  if (!getActiveUserId()) {
+    return { theme: 'auto', colorScheme: 'auto', persisted: false }
+  }
+
+  return apiJson<AppearancePrefs>(API_BASE)
+}
+
+export async function saveAppearancePrefsToServer(theme: string, colorScheme: string): Promise<void> {
+  if (!getActiveUserId()) return
+
+  await apiJson<AppearancePrefs>(API_BASE, {
+    method: 'PUT',
+    body: JSON.stringify({ theme, colorScheme })
+  })
+}
+
+/** Merge server-stored appearance with local cache (server wins after cache wipe). */
+export async function syncAppearancePrefs(userId?: string | null): Promise<void> {
+  const id = userId?.trim() || getActiveUserId()
+  if (!id) return
+
+  try {
+    const server = await fetchAppearancePrefs()
+
+    if (server.persisted) {
+      setThemePreference(id, server.theme)
+      setColorSchemePreference(id, server.colorScheme)
+    } else if (hasLocalAppearancePrefs(id)) {
+      await saveAppearancePrefsToServer(getThemePreference(id), getColorSchemePreference(id))
+    }
+  } catch (err) {
+    console.warn('Failed to sync appearance preferences:', err)
+  }
+
+  notifyAppearanceChanged()
+}