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/web.rs

@@ -61,6 +61,17 @@ pub struct JobSnapshot {
     pub state: &'static str,
     pub progress: f32,
     pub last_line: String,
+    /// Classification of the failure, if `state == "failed"`. One of
+    /// `rate-limited`, `members-only`, `geo-blocked`, `not-found`,
+    /// `codec-missing`, `disk-full`, `network-error`, `bad-cookies`, `other`,
+    /// or `null` while still running / on success. Drives the suggested
+    /// action hint in the UI.
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub error_class: Option<crate::error_class::ErrorClass>,
+    /// Human-readable one-line suggested action paired with `error_class`.
+    /// Empty when `error_class` is `Other` or `None`.
+    #[serde(skip_serializing_if = "str::is_empty")]
+    pub error_hint: &'static str,
 }
 
 /// All mutable state shared across axum handlers via `Arc<WebState>`.
@@ -145,6 +156,12 @@ impl WebState {
                 },
                 progress: j.progress,
                 last_line: j.log.back().cloned().unwrap_or_default(),
+                // Skip `Other` so the badge doesn't get a useless generic
+                // label — the raw log line is still shown for that case.
+                error_class: j.failure_class.filter(|c|
+                    *c != crate::error_class::ErrorClass::Other
+                ),
+                error_hint: j.failure_class.map(|c| c.hint()).unwrap_or(""),
             })
             .collect()
     }