feat: name downloaded file after the video title and add a back-to-home link

The download endpoint served files under their internal uuid-based
storage name; it now builds the Content-Disposition filename from the
probed video title (persisted on submit) with the original extension,
falling back to the old behavior when no title is available. The
status page also gets an always-visible link back to the homepage.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

messages/en.json

@@ -28,6 +28,7 @@
     "videoLength": "Length: {duration}"
   },
   "status": {
+    "backHome": "Back to homepage",
     "heading": "Download status",
     "notFound": "Download not found.",
     "loading": "Loading...",