ci: make clippy a hard gate

The advisory step existed because the tree carried a warning backlog. Measured
on 1.97.1 — the pinned toolchain CI actually uses — that backlog is three
warnings, not the ~51 the comment claimed: two unnecessary_sort_by in
smart_cache and one redundant into_iter in offline. Fixed, so clippy now runs
with -D warnings and a warning means new breakage.

Worth recording why this took a toolchain pin to do safely: the same tree
measured 0 warnings on 1.92.0 and 3 on 1.97.1. Flipping the flag on a local
measurement, without the pin, would have reddened CI on the next push.

TRACES: | DR-206

src-tauri/src/commands/download/smart_cache.rs

@@ -187,7 +187,7 @@ pub async fn get_album_recommendations(
     }
 
     // Sort by tracks played (descending)
-    recommendations.sort_by(|a, b| b.tracks_played.cmp(&a.tracks_played));
+    recommendations.sort_by_key(|r| std::cmp::Reverse(r.tracks_played));
 
     Ok(recommendations)
 }
@@ -224,7 +224,7 @@ pub fn get_album_affinity_status(
         .collect();
 
     // Sort by play count (descending)
-    statuses.sort_by(|a, b| b.unique_tracks_played.cmp(&a.unique_tracks_played));
+    statuses.sort_by_key(|s| std::cmp::Reverse(s.unique_tracks_played));
 
     Ok(statuses)
 }