Classify yt-dlp failures into actionable buckets (2.3)

When a job fails, today the user sees a raw stderr line — often
something like "ERROR: [youtube] dQw4w9WgXcQ: Sign in to confirm
you're not a bot" that doesn't tell a non-expert what to do.

New src/error_class.rs scans the failed job's log buffer for the
nine well-known yt-dlp fingerprints and returns:
- an ErrorClass enum (rate-limited, members-only, geo-blocked,
  not-found, codec-missing, disk-full, network-error, bad-cookies,
  other)
- a one-line human-readable suggested action paired with each class

Auto-populated in Job::drain when state transitions to Failed.
Exposed on JobSnapshot as error_class + error_hint. Rendered in
the web UI Downloads modal (red badge + hint box) and the desktop
downloads panel (colored label + hint text).

Other (no fingerprint matched) is filtered out of the badge so the
existing raw last_line keeps doing the talking for unknown failures.

Includes 10 unit tests covering each pattern + the walks-from-end
priority case.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/main.rs

@@ -18,6 +18,7 @@ mod config;
 mod database;
 mod download_options;
 mod downloader;
+mod error_class;
 mod library;
 mod maintenance;
 mod platform;