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/i18n/locales/en/downloads.json

@@ -206,6 +206,49 @@
       "empty_edit": "Choose a trim range or crop area first.",
       "source_not_ready": "The source download isn't ready to edit.",
       "generic": "That edit couldn't be created."
+    },
+    "fail": {
+      "title": "Download failed",
+      "detailsLink": "details",
+      "detailsToggle": "Technical details",
+      "cat": {
+        "unavailable": {
+          "short": "Video unavailable",
+          "body": "This video is private, deleted, or otherwise no longer available at the source, so it can't be downloaded."
+        },
+        "login_required": {
+          "short": "Sign-in required",
+          "body": "This video needs a signed-in, age-verified, or members-only account to view, so it can't be downloaded here."
+        },
+        "geo_blocked": {
+          "short": "Region-blocked",
+          "body": "The source blocks this video in the server's region, so it can't be downloaded."
+        },
+        "format_unavailable": {
+          "short": "No downloadable format",
+          "body": "No downloadable video format was offered for this link — it may be images-only or an unsupported page."
+        },
+        "drm": {
+          "short": "DRM-protected",
+          "body": "This video is DRM-protected and cannot be downloaded."
+        },
+        "postprocess": {
+          "short": "Processing failed",
+          "body": "The video downloaded but a processing step (merge or convert) failed. A retry often fixes this."
+        },
+        "source_missing": {
+          "short": "Source unavailable",
+          "body": "The original download this clip is based on is no longer available."
+        },
+        "network": {
+          "short": "Network error",
+          "body": "A network error interrupted the download. A retry usually fixes this."
+        },
+        "unknown": {
+          "short": "Download failed",
+          "body": "Something went wrong while downloading this item. The technical details below may help."
+        }
+      }
     }
   }
 }