feat(downloads): categorize download failures + a readable error dialog (S2)

Failed downloads stored the raw yt-dlp/ffmpeg text and dumped it as a truncated red
one-liner in the queue. Classify the failure into a category (unavailable / login_required /
geo_blocked / format_unavailable / drm / postprocess / source_missing / network / unknown)
and surface it properly:

- backend: app/downloads/errors.classify maps the raw message to a category code; the worker
  stores it in the new download_jobs.error_code (migration 0061) at both failure sites; resume
  clears it; the serializer exposes it. Raw text kept as technical detail.
- frontend: the queue row shows a clean localized reason + a "details" link opening a modal
  with the explanation, the raw text behind a disclosure, and a Retry only for transient
  categories (network/postprocess/unknown).
- tests: classify() unit coverage over verbatim yt-dlp/ffmpeg message shapes.

backend/app/routes/downloads.py

@@ -81,6 +81,7 @@ def _serialize(job: DownloadJob, asset: MediaAsset | None) -> dict:
         "eta_s": job.eta_s,
         "phase": job.phase,
         "error": job.error,
+        "error_code": job.error_code,
         "queue_pos": job.queue_pos,
         "created_at": iso(job.created_at),
         "source_kind": job.source_kind,
@@ -468,6 +469,7 @@ def resume_download(
     if job.status in ("paused", "error"):
         job.progress = 0
         job.error = None
+        job.error_code = None
         # If the shared asset itself failed, reset it to pending so the worker actually
         # re-downloads — otherwise the requeued job would instantly re-inherit the asset's
         # stale error (the worker short-circuits a job whose asset is already errored).