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]>

modules/app_dash.py

@@ -559,17 +559,19 @@ def import_dashcam(
                 "message": f"Failed to mount {device_path}.",
             }
 
-        try:
-            summary = copy_files(source, destination_folder, relative_paths)
-            summary["status"] = (
-                "invalid_selection" if summary.pop("invalid_selection", False) else "ok"
-            )
-            if summary["status"] == "invalid_selection":
-                summary["message"] = summary["errors"][0]
-            summary["device"] = device_path
-            return summary
-        finally:
-            if mounted_here:
+    # Lock released — copy runs without blocking other requests
+    try:
+        summary = copy_files(source, destination_folder, relative_paths)
+        summary["status"] = (
+            "invalid_selection" if summary.pop("invalid_selection", False) else "ok"
+        )
+        if summary["status"] == "invalid_selection":
+            summary["message"] = summary["errors"][0]
+        summary["device"] = device_path
+        return summary
+    finally:
+        if mounted_here:
+            with _device_lock:
                 unmount_device(mount_point)