Offer the previous session of the same sport from the workout page

"How does this compare to last time?" is the most likely next question
after reading a workout, and answering it meant going back to the list and
scanning for the one before.

Resolve the previous session server-side, through the same canonical picker
the list uses, so the link can never land on a duplicate row recorded by a
second watch, and render it beside the own-history average line. Null when
nothing older exists, and then no link is shown.

The stats card's hand-rolled heading moves to the icon-less TileHeader
while the file is open.

docs/api/openapi.yaml

@@ -20073,6 +20073,10 @@ components:
             - type: "null"
         canonicalId:
           type: string
+        previousWorkoutId:
+          anyOf:
+            - type: string
+            - type: "null"
       required:
         - id
         - sportType
@@ -20099,6 +20103,7 @@ components:
         - sportContext
         - aiInsight
         - canonicalId
+        - previousWorkoutId
       additionalProperties: false
     WorkoutRouteGeometry:
       type: object