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.

frontend/src/i18n/locales/de/downloads.json

@@ -37,6 +37,12 @@
     "canceled": "Abgebrochen",
     "expired": "Abgelaufen"
   },
+  "phase": {
+    "video": "Video wird geladen",
+    "audio": "Audio wird geladen",
+    "merging": "Zusammenführen",
+    "processing": "Verarbeitung"
+  },
   "actions": {
     "pause": "Pause",
     "resume": "Fortsetzen",