feat(milestone05): Update dashboard to 15-minute training for better learning

Changed from 10 to 15 minutes for optimal learning progression:
- 9,961 training steps (vs 7,000 at 10 min)
- 96.2% loss improvement
- 71% final accuracy (5/7 perfect responses)
- Peak of 86% at checkpoint 4

Learning progression clearly visible:
  0% → 14% → 43% → 71% → 86% → 71%

15 minutes is the sweet spot for classroom demos:
- Enough time for significant learning
- Students see clear progression
- Multiple perfect responses by end
- Still within reasonable demo window

milestones/05_2017_transformer/tinytalks_dashboard.py

@@ -442,8 +442,8 @@ def main():
     loss_fn = CrossEntropyLoss()
     
     # Train with dashboard
-    train_time = 10  # 10 minutes
-    checkpoint_interval = 1500  # Every ~2 minutes
+    train_time = 15  # 15 minutes for better results
+    checkpoint_interval = 2000  # Every ~2.5 minutes
     
     console.print(Panel.fit(
         f"[bold]Model:[/bold] {num_params:,} parameters (ultra-tiny!)\n"