feat: add smart "Workout Now" recommendation to workout picker

Auto-recommends the next workout in the 4-day cycle based on the
user's last completed session (one with logged sets). Redesigns
the /workouts page with highlighted recommendation card and
renames the back button to "Change Workout".

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/services/workout_session_service.py

@@ -10,6 +10,7 @@ from typing import Optional
 import structlog
 from sqlmodel import Session, select
 
+from app.models.workout_log import WorkoutLog
 from app.models.workout_session import WorkoutSession
 
 logger = structlog.get_logger(__name__)
@@ -71,6 +72,17 @@ class WorkoutSessionService:
         )
         return ws
 
+    def get_last_completed_session(self, user_id: int) -> Optional[WorkoutSession]:
+        """Get the most recent session that has at least one logged set."""
+        statement = (
+            select(WorkoutSession)
+            .join(WorkoutLog, WorkoutLog.session_id == WorkoutSession.id)
+            .where(WorkoutSession.user_id == user_id)
+            .order_by(WorkoutSession.date.desc())
+            .limit(1)
+        )
+        return self._session.exec(statement).first()
+
     def list_sessions(
         self,
         user_id: int,