fix(update): stop blocking event loop during rebuild + fix infinite spinner

The update endpoint ran the entire git pull + docker build (up to 10 min)
synchronously inside the request handler, blocking the whole server for
everyone while it ran. Separately, the frontend spinner was only hidden on
error, never on success, so it spun forever even when the update worked.

- Run the update in a background thread; the request returns immediately
- Add GET /settings/update/status for progress polling (backup/pull/build/restart)
- Frontend polls status, then waits for the app to come back after the
  container restart, and shows a clear done/timeout message instead of an
  endless spinner

Co-Authored-By: Claude Sonnet 5 <[email protected]>

static/js/app.js

@@ -1374,11 +1374,12 @@ async function loadVersionInfo() {
 
         if (needsUpdate) {
             html += `<div class="mt-3">
-                <button class="btn btn-warning" onclick="triggerUpdate()">
+                <button class="btn btn-warning" id="update-trigger-btn" onclick="triggerUpdate()">
                     <span class="spinner-border spinner-border-sm d-none me-1" id="update-spinner"></span>
                     <i class="bi bi-arrow-repeat me-1"></i>${t('settings.triggerUpdate')}
                 </button>
                 <div class="text-muted small mt-1">${t('settings.updateWarning')}</div>
+                <div class="small mt-2 d-none" id="update-progress-text"></div>
             </div>`;
         }
         el.innerHTML = html;
@@ -1390,14 +1391,76 @@ async function loadVersionInfo() {
 async function triggerUpdate() {
     if (!confirm(t('settings.confirmUpdate'))) return;
     const spinner = document.getElementById('update-spinner');
+    const btn = document.getElementById('update-trigger-btn');
+    const progressText = document.getElementById('update-progress-text');
+    const setProgress = (msg) => {
+        if (!progressText) return;
+        progressText.classList.remove('d-none');
+        progressText.textContent = msg;
+    };
+    const stopUpdateUi = () => {
+        if (spinner) spinner.classList.add('d-none');
+        if (btn) btn.disabled = false;
+    };
+
     if (spinner) spinner.classList.remove('d-none');
+    if (btn) btn.disabled = true;
+    setProgress(t('settings.updateStepStarting'));
+
     try {
         const data = await api('POST', '/settings/update');
         showSettingsAlert('success', data.message || t('messages.updateStarted'));
     } catch (err) {
         showSettingsAlert('danger', t('errors.failed', { error: err.message }));
-        if (spinner) spinner.classList.add('d-none');
+        stopUpdateUi();
+        return;
     }
+
+    // Phase 1: poll build/pull progress until the container restarts
+    // (connection drops, which is expected and is our cue to move to phase 2).
+    const stepLabelKey = {
+        backup: 'settings.updateStepBackup',
+        pull: 'settings.updateStepPull',
+        build: 'settings.updateStepBuild',
+        restart: 'settings.updateStepRestart',
+    };
+    for (let i = 0; i < 200; i++) {
+        await new Promise(r => setTimeout(r, 2000));
+        try {
+            const st = await api('GET', '/settings/update/status');
+            if (st.state === 'failed') {
+                showSettingsAlert('danger', st.message || t('errors.requestFailed'));
+                stopUpdateUi();
+                return;
+            }
+            setProgress(t(stepLabelKey[st.step] || 'settings.updateStepStarting') + (st.message ? ` — ${st.message}` : ''));
+        } catch (err) {
+            // Connection dropped — the container is very likely mid-restart. Move on.
+            break;
+        }
+    }
+
+    // Phase 2: wait for the app to come back up, then reload version info.
+    setProgress(t('settings.updateStepReconnecting'));
+    for (let i = 0; i < 90; i++) {
+        await new Promise(r => setTimeout(r, 2000));
+        try {
+            await api('GET', '/settings/version');
+            setProgress(t('settings.updateStepDone'));
+            stopUpdateUi();
+            showSettingsAlert('success', t('settings.updateStepDone'));
+            await loadVersionInfo();
+            return;
+        } catch (err) {
+            // still restarting — keep polling
+        }
+    }
+
+    // Gave up waiting — surface this instead of spinning forever.
+    stopUpdateUi();
+    setProgress('');
+    if (progressText) progressText.classList.add('d-none');
+    showSettingsAlert('warning', t('settings.updateStepTimeout'));
 }
 
 // ---------------------------------------------------------------------------