fix(sync): guard the Sync-now button against concurrent runs

Clicking Sync twice started a second concurrent runSyncNow, and the click
event was passed as runSyncNow's `_attempt` retry counter (so the "Syncing…"
toast and 412-retry bound were both broken). Wrapped the handler: disable the
button while a sync runs, and call runSyncNow() with no arg. Internal retries
are unaffected.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

js/app.js

@@ -8938,7 +8938,15 @@ async function init() {
         Bridge.setPref(SYNC_PREFS.preBackup, e.target.checked ? '1' : ''));
     if (syncSetPwd) syncSetPwd.addEventListener('click', syncSetEncPwdFlow);
     if (syncTest)   syncTest.addEventListener('click', syncTestConnection);
-    if (syncNow)    syncNow.addEventListener('click', runSyncNow);
+    // Guard the user entry point: ignore clicks while a sync is running (no
+    // concurrent syncs) and call runSyncNow() with NO arg — the click event
+    // must not land in its `_attempt` retry counter. Internal retries call
+    // runSyncNow directly, so they're unaffected.
+    if (syncNow) syncNow.addEventListener('click', async () => {
+        if (syncNow.disabled) return;
+        syncNow.disabled = true;
+        try { await runSyncNow(); } finally { syncNow.disabled = false; }
+    });
 
     $('#settingAutoBackupEnabled').addEventListener('change', onToggleAutoBackup);
     $('#autoBackupPickDirBtn').addEventListener('click', pickAutoBackupFolder);