fix(dashcam): prevent site freeze during import/convert

- _convert_bg now runs in real threading.Thread (not gevent greenlet)
  so ffmpeg proc.wait() doesn't block gevent event loop
- _device_lock released before copy loop; only held for mount/unmount
  so /api/dash/files and other routes don't block during long copies
- Date now shows time (Aug 9, 10:30 PM) on mobile video rows
- Spacing added above converting spinner badge

Co-authored-by: Copilot <[email protected]>

static/js/local-videos.js

@@ -115,7 +115,7 @@
             // Date shown only on mobile (desktop has its own column)
             const mobileMeta = document.createElement("div");
             mobileMeta.className = "dash-thumb-meta text-muted";
-            mobileMeta.textContent = new Date(file.modified_at * 1000).toLocaleDateString(undefined, { month: 'short', day: 'numeric' });
+            mobileMeta.textContent = new Date(file.modified_at * 1000).toLocaleString(undefined, { month: 'short', day: 'numeric', hour: '2-digit', minute: '2-digit' });
             preview.appendChild(mobileMeta);
             // Converting badge shown on mobile inside thumbnail cell
             if (file.converting) {