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/models.py

@@ -877,7 +877,8 @@ class DownloadJob(Base, TimestampMixin, UpdatedAtMixin):
     speed_bps: Mapped[int | None] = mapped_column(BigInteger)
     eta_s: Mapped[int | None] = mapped_column(Integer)
     phase: Mapped[str | None] = mapped_column(String(32))
-    error: Mapped[str | None] = mapped_column(Text)
+    error: Mapped[str | None] = mapped_column(Text)  # raw yt-dlp/ffmpeg text (technical detail)
+    error_code: Mapped[str | None] = mapped_column(String(24))  # category, see downloads.errors.classify
     queue_pos: Mapped[int] = mapped_column(Integer, default=0, server_default="0")