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/downloadModals.tsx

@@ -8,6 +8,7 @@ import { api, type DownloadJob } from "../lib/api";
 import { inputCls } from "./ui/form";
 import { Button } from "./ui/Button";
 import { IconBtn } from "./downloadRow";
+import { failCopyKeys } from "../lib/downloadFailure";
 
 // Per-item user modals split out of DownloadCenter (self-contained, prop-driven): rename/metadata
 // editing and the failure-detail window.
@@ -142,8 +143,8 @@ export function ErrorModal({
   return (
     <Modal title={t("downloads.errors.fail.title")} onClose={onClose}>
       <div className="space-y-3">
-        <div className="font-medium">{t(`downloads.errors.fail.cat.${code}.short`)}</div>
-        <p className="text-sm text-muted">{t(`downloads.errors.fail.cat.${code}.body`)}</p>
+        <div className="font-medium">{t(failCopyKeys(job, "short"))}</div>
+        <p className="text-sm text-muted">{t(failCopyKeys(job, "body"))}</p>
         {job.error && (
           <div>
             <button