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/downloader.rs

@@ -128,6 +128,11 @@ pub struct Job {
     pub progress: f32,
     /// Rolling log buffer — capped at [`JOB_LOG_CAP`] lines via O(1) front-pop.
     pub log: VecDeque<String>,
+    /// Best-effort classification of the failure, populated when `state`
+    /// transitions to `Failed`. `None` while running or on success. The UI
+    /// surfaces the class + a one-line suggested fix from
+    /// [`crate::error_class`].
+    pub failure_class: Option<crate::error_class::ErrorClass>,
     rx: Receiver<Msg>,
 }
 
@@ -144,6 +149,15 @@ impl Job {
                 Msg::Progress(p) => self.progress = p,
                 Msg::Finished(ok) => {
                     self.state = if ok { JobState::Done } else { JobState::Failed };
+                    // Classify only on the failure transition so the
+                    // classifier doesn't re-run for every poll() call on a
+                    // long-finished job. The log is already in `self.log`
+                    // by this point since we drained Line messages above.
+                    if !ok && self.failure_class.is_none() {
+                        self.failure_class = Some(crate::error_class::classify(
+                            self.log.iter().map(|s| s.as_str()),
+                        ));
+                    }
                 }
             }
         }
@@ -594,7 +608,15 @@ impl Downloader {
             let _ = tx.send(Msg::Finished(ok));
         });
 
-        self.jobs.push(Job { url, label, state: JobState::Running, progress: 0.0, log: VecDeque::new(), rx });
+        self.jobs.push(Job {
+            url,
+            label,
+            state: JobState::Running,
+            progress: 0.0,
+            log: VecDeque::new(),
+            failure_class: None,
+            rx,
+        });
     }
 
     /// Drain pending messages from all job threads and promote queued jobs.