feat(M003/S03): play count column + data pipeline

Backend:
- GetAllTracksWithFullMetadata queries now select play_count and last_played
- mapTrackRow accepts and passes through PlayCount/LastPlayed
- PlayCount + LastPlayed added to library.Track struct
- sqlcgen Row types updated with new fields

Frontend:
- PlayCount + LastPlayed added to library.Track TypeScript model
- 'Plays' column added to track-list column definitions (60px, right-aligned, sortable)

Queries without play data (search, genre, album) pass 0/empty defaults.

frontend/wailsjs/go/models.ts

@@ -266,6 +266,8 @@ export namespace library {
 	    Channels: number;
 	    Bitrate: number;
 	    FileSize: number;
+	    PlayCount: number;
+	    LastPlayed: string;
 	
 	    static createFrom(source: any = {}) {
 	        return new Track(source);
@@ -289,6 +291,8 @@ export namespace library {
 	        this.Channels = source["Channels"];
 	        this.Bitrate = source["Bitrate"];
 	        this.FileSize = source["FileSize"];
+	        this.PlayCount = source["PlayCount"];
+	        this.LastPlayed = source["LastPlayed"];
 	    }
 	}