feat: add German and English UI language support

A single-language interface makes the app less usable for part of the audience
and encourages more hardcoded copy as new screens and states are added.
Introducing a shared i18n path with a persisted language preference gives the
UI one consistent way to present text and lowers the risk of partial or
inconsistent localization over time.

The surrounding copy cleanup belongs in the same change because it aligns
existing labels, messages, and settings text with that translation structure
instead of leaving mixed localization approaches in place.

src/main/index.ts

@@ -111,7 +111,7 @@ app.whenReady().then(async () => {
   await auth.init()
   const helix = new HelixClient(clientId, async () => {
     const token = await auth.getValidAccessToken()
-    if (!token) throw new Error('Kein gueltiges Twitch-Access-Token vorhanden')
+    if (!token) throw new Error('No valid Twitch access token available')
     return token
   })
   const playback = new PlaybackService()
@@ -126,7 +126,7 @@ app.whenReady().then(async () => {
   })
 
   if (!clientId) {
-    log.warn('[twitch4steamdeck] MAIN_VITE_TWITCH_CLIENT_ID nicht gesetzt — Login wird im UI als nicht konfiguriert angezeigt.')
+    log.warn('[twitch4steamdeck] MAIN_VITE_TWITCH_CLIENT_ID is not set — the UI will show login as not configured.')
   }
 
   createWindow()