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.

frontend/src/lib/api/downloads.ts

@@ -69,7 +69,8 @@ export interface DownloadJob {
   speed_bps: number | null;
   eta_s: number | null;
   phase: string | null;
-  error: string | null;
+  error: string | null; // raw yt-dlp/ffmpeg text (technical detail)
+  error_code: string | null; // failure category, see backend downloads.errors.classify
   queue_pos: number;
   created_at: string | null;
   source_kind: string;