Integrate ExerciseDB v2 data layer (muscleMap.ts, exercisedb.ts) to enrich
the 77 static exercises with detailed muscle targeting, similar exercises,
and expand the catalog to 254 exercises. Add interactive SVG muscle body
diagrams for both the stats page heatmap and exercise list filtering, with
split front/back views flanking the exercise list on desktop. Replace body
part dropdown with unified muscle group multi-select with pill tags.
