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.test.ts

@@ -0,0 +1,72 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+import {
+  fetchAppearancePrefs,
+  saveAppearancePrefsToServer,
+  syncAppearancePrefs
+} from './appearancePrefs.js'
+import { setThemePreference } from './userPreferences.js'
+
+const USER_ID = 'appearance-sync-user'
+
+vi.mock('./api.js', () => ({
+  apiJson: vi.fn()
+}))
+
+import { apiJson } from './api.js'
+
+const mockedApiJson = vi.mocked(apiJson)
+
+describe('appearancePrefs', () => {
+  beforeEach(() => {
+    localStorage.clear()
+    vi.clearAllMocks()
+  })
+
+  it('fetchAppearancePrefs returns defaults when not authenticated', async () => {
+    await expect(fetchAppearancePrefs()).resolves.toEqual({
+      theme: 'auto',
+      colorScheme: 'auto',
+      persisted: false
+    })
+    expect(mockedApiJson).not.toHaveBeenCalled()
+  })
+
+  it('syncAppearancePrefs applies server prefs after cache wipe', async () => {
+    localStorage.setItem('active_userid', USER_ID)
+    mockedApiJson.mockResolvedValueOnce({
+      theme: 'ocean',
+      colorScheme: 'dark',
+      persisted: true
+    })
+
+    const changed = vi.fn()
+    window.addEventListener('appearance-changed', changed)
+
+    await syncAppearancePrefs(USER_ID)
+
+    expect(localStorage.getItem(`user_pref_theme_${USER_ID}`)).toBe('ocean')
+    expect(localStorage.getItem(`user_pref_color_scheme_${USER_ID}`)).toBe('dark')
+    expect(changed).toHaveBeenCalledTimes(1)
+  })
+
+  it('syncAppearancePrefs uploads local prefs when server has none', async () => {
+    localStorage.setItem('active_userid', USER_ID)
+    setThemePreference(USER_ID, 'material')
+    mockedApiJson
+      .mockResolvedValueOnce({ theme: 'auto', colorScheme: 'auto', persisted: false })
+      .mockResolvedValueOnce({ theme: 'material', colorScheme: 'auto', persisted: true })
+
+    await syncAppearancePrefs(USER_ID)
+
+    expect(mockedApiJson).toHaveBeenCalledTimes(2)
+    expect(mockedApiJson).toHaveBeenLastCalledWith('/api/auth/appearance-prefs', {
+      method: 'PUT',
+      body: JSON.stringify({ theme: 'material', colorScheme: 'auto' })
+    })
+  })
+
+  it('saveAppearancePrefsToServer skips when not authenticated', async () => {
+    await saveAppearancePrefsToServer('ocean', 'light')
+    expect(mockedApiJson).not.toHaveBeenCalled()
+  })
+})