feat(downloads): clear multi-phase download progress + transient-error retries

The progress bar looked broken (95% -> 5% -> recount) because yt-dlp downloads the video and
audio streams separately (each 0->100%) then merges — with no indication of which step is
running. Now:
- worker labels the phase from the stream codecs (video / audio) and via a postprocessor hook
  (merging / processing), so the user sees what's happening
- DownloadCenter shows the phase as the status and renders an indeterminate pulse (no bogus %)
  during merge/processing; phase i18n en/hu/de
- yt-dlp retries (3) + fragment_retries (5) so transient network blips self-heal instead of
  failing the job (seen once as a DNS 'No address associated with hostname' error)

Verified: phase transitions queued -> video -> processing -> done.

backend/app/downloads/formats.py

@@ -91,6 +91,10 @@ def build_ydl_opts(spec: dict, outtmpl: str, progress_hook) -> dict:
         "writethumbnail": True,  # kept for the poster.jpg sidecar (and embed, if requested)
         "progress_hooks": [progress_hook],
         "postprocessors": [],
+        # Self-heal transient network blips instead of failing the whole job.
+        "retries": 3,
+        "fragment_retries": 5,
+        "retry_sleep": {"http": 2, "fragment": 2},
     }
 
     if s["mode"] == "a":