feat(downloads): a per-site failure catalog — Facebook's walls say why and what to do

A failed Facebook download read "Download failed": yt-dlp words the login wall
(18+ reel, closed group) as "Cannot parse data", which no category matched.
Measured on dev with the owner's cookie before writing a rule — the same text is
the login wall without a cookie and a refused cookie with one.

- errors.diagnose(msg, url, cookie_attached) tries a host-scoped catalog first
  (all-of markers, cookie state, link fragment) and falls back to the category.
  First entries: facebook.login_wall, facebook.cookie_rejected,
  facebook.group_post_video. The module doc is the recipe for the next entry.
- download_jobs.error_detail (0084) + backfill of the stored Facebook parse
  failures that were unknown.
- The pipeline's failure branch passes the link and whether the cookie went
  with it; resume and redownload clear code and detail (redownload never
  cleared error_code).
- A job meeting an already-failed asset got the text but no code, so it read
  "Download failed" where the first job said why. It is diagnosed again, with
  the cookie state from credentials.is_authed_asset (now shared with the
  cookie gate); the redacted network text classifies back to network.
- FE: failCopyKeys gives the row and the failure window one lookup — catalog
  copy, else the category. Copy in EN and HU; a contract test pins every
  backend entry to both locales.

frontend/src/components/downloadRow.tsx

@@ -9,6 +9,7 @@ import { notify } from "../lib/notifications";
 import { copyText } from "../lib/clipboard";
 import { formatBytes, formatDuration, formatEta, formatSpeed, relativeTime } from "../lib/format";
 import { navItemAttrs } from "../lib/listNav";
+import { failCopyKeys } from "../lib/downloadFailure";
 
 // Download-row presentation + the small shared primitives (status pill, icon button, usage bar),
 // split out of DownloadCenter (all purely prop-driven, no shared state). Rendered by the queue,
@@ -260,7 +261,7 @@ export function DownloadRow({
               className="text-danger hover:opacity-80 inline-flex items-center gap-1 truncate"
               title={t("downloads.errors.fail.title")}
             >
-              · {t(`downloads.errors.fail.cat.${job.error_code || "unknown"}.short`)}
+              · {t(failCopyKeys(job, "short"))}
               <span className="text-muted underline">{t("downloads.errors.fail.detailsLink")}</span>
             </button>
           ) : null}