fix(notifications): print-complete duration uses actual elapsed, not slicer estimate (#1198)

Pre-fix, _background_notifications in main.py:3434 built archive_data
  with print_time_seconds (the slicer's pre-print estimate parsed from
  the 3MF at archive creation), and notification_service.py:909 formatted
  that field straight into the {{duration}} template variable. A print
  cancelled 2 minutes into a 3-hour estimate notified "duration: 3h".

  Compute actual_time_seconds from started_at/completed_at in main.py and
  add it to archive_data. notification_service.py prefers it, falls back
  to print_time_seconds when the actual can't be derived.

  Also add "cancelled" to the list of statuses that get completed_at set
  in update_archive_status — pre-fix only completed/failed/aborted got a
  timestamp, so queue-UI cancellations had no actual elapsed to compute
  from. Audited every completed_at consumer; none depend on NULL to mean
  "cancelled" (status field already carries that signal), and the
  statistics-totals aggregation gets more accurate too as a side effect.

  3 new regression tests in TestNotificationVariableFallbacks pin the
  {{duration}} variable contract (actual wins over estimate; estimate
  falls in when actual is missing; "Unknown" when both absent).

static/index.html

@@ -26,7 +26,7 @@
 
     <!-- Splash screens for iOS -->
     <link rel="apple-touch-startup-image" href="./img/android-chrome-512x512.png" />
-    <script type="module" crossorigin src="./assets/index-CrzRt--w.js"></script>
+    <script type="module" crossorigin src="./assets/index-D7M74u_C.js"></script>
     <link rel="stylesheet" crossorigin href="./assets/index-Cw7zekS6.css">
   </head>
   <body>