feat: migrate frontend app to nextjs app router

Signed-off-by: Tilman Vatteroth <git@tilmanvatteroth.de>

frontend/src/components/application-loader/application-loader.tsx

@@ -1,3 +1,4 @@
+'use client'
 /*
  * SPDX-FileCopyrightText: 2022 The HedgeDoc developers (see AUTHORS file)
  *

frontend/src/components/application-loader/initializers/load-dark-mode.ts

@@ -6,7 +6,6 @@
 import { DARK_MODE_LOCAL_STORAGE_KEY } from '../../../hooks/dark-mode/use-save-dark-mode-preference-to-local-storage'
 import { setDarkModePreference } from '../../../redux/dark-mode/methods'
 import { DarkModePreference } from '../../../redux/dark-mode/types'
-import { isClientSideRendering } from '../../../utils/is-client-side-rendering'
 import { Logger } from '../../../utils/logger'
 
 const logger = new Logger('Dark mode initializer')
@@ -29,9 +28,6 @@ export const loadDarkMode = (): Promise<void> => {
  *         {@link false} if the user doesn't prefer dark mode or if the value couldn't be read from local storage.
  */
 const fetchDarkModeFromLocalStorage = (): DarkModePreference => {
-  if (!isClientSideRendering()) {
-    return DarkModePreference.AUTO
-  }
   try {
     const colorScheme = window.localStorage.getItem(DARK_MODE_LOCAL_STORAGE_KEY)
     if (colorScheme === 'dark') {

frontend/src/components/application-loader/initializers/setupI18n.ts

@@ -36,6 +36,9 @@ export const setUpI18n = async (): Promise<void> => {
       }
     })
 
-  i18n.on('languageChanged', (language) => (Settings.defaultLocale = language))
+  i18n.on('languageChanged', (language) => {
+    Settings.defaultLocale = language
+    document.documentElement.lang = i18n.language
+  })
   Settings.defaultLocale = i18n.language
 }