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/templates/pages/workout_day.html

@@ -27,5 +27,5 @@
     {% endfor %}
 </section>
 
-<a href="/workouts" role="button" class="outline">Back to All Days</a>
+<a href="/workouts" role="button" class="outline">Change Workout</a>
 {% endblock %}