Add frontend git update button and backend update endpoint

public/app.js

@@ -10,6 +10,7 @@ const trackList = document.getElementById('trackList');
 const sendBtn = document.getElementById('sendBtn');
 const cleanupToggle = document.getElementById('cleanupToggle');
 const updateLibraryBtn = document.getElementById('updateLibraryBtn');
+const updateFrontendBtn = document.getElementById('updateFrontendBtn');
 
 let selectedAlbum = null;
 
@@ -239,6 +240,29 @@ async function updateLibrary() {
   }
 }
 
+async function updateFrontendFromGit() {
+  setStatus('Hole aktuelle Git-Daten...');
+  updateFrontendBtn.disabled = true;
+
+  try {
+    const data = await fetchJson('/api/system/update-frontend', {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json' }
+    });
+
+    if (data.updated) {
+      setStatus(`Frontend aktualisiert (${data.before} -> ${data.after}). Seite wird neu geladen...`);
+      setTimeout(() => window.location.reload(), 1200);
+    } else {
+      setStatus(`Bereits aktuell (${data.after}).`);
+    }
+  } catch (err) {
+    setStatus(`Frontend-Update fehlgeschlagen: ${err.message}`, true);
+  } finally {
+    updateFrontendBtn.disabled = false;
+  }
+}
+
 searchBtn.addEventListener('click', searchSpotify);
 queryInput.addEventListener('keydown', (event) => {
   if (event.key === 'Enter') {
@@ -247,3 +271,4 @@ queryInput.addEventListener('keydown', (event) => {
 });
 sendBtn.addEventListener('click', sendToLidarr);
 updateLibraryBtn.addEventListener('click', updateLibrary);
+updateFrontendBtn.addEventListener('click', updateFrontendFromGit);