fix: persist and display Volume PRs in workout history

Volume PRs were calculated client-side in the workout summary but never
saved to the database, so they didn't appear in history detail pages.
Add bestSetVolume PR detection to both session save and recalculate
endpoints, and render the new type in the history detail view.

src/models/WorkoutSession.ts

@@ -31,7 +31,7 @@ export interface ICompletedExercise {
 
 export interface IPr {
   exerciseId: string;
-  type: string; // 'est1rm' | 'maxWeight' | 'repMax'
+  type: string; // 'est1rm' | 'maxWeight' | 'bestSetVolume' | 'repMax'
   value: number;
   reps?: number;
 }