feat: show real-time download progress bar for app updates

Leverage the Tauri updater plugin's DownloadEvent callback to display
a progress bar with downloaded/total bytes during app updates, replacing
the previous spinner-only feedback.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/i18n/messages/de.json

@@ -172,6 +172,7 @@
     "checking": "Wird geprüft...",
     "checkUpdate": "Nach Updates suchen",
     "updating": "Wird installiert...",
+    "downloading": "Herunterladen...",
     "upgradeTo": "Auf v{version} aktualisieren",
     "foundUpdate": "Neue Version v{version} gefunden",
     "alreadyLatest": "Du verwendest bereits die neueste Version",