Add training loss chart visualization

[provandal]

The app already tracks epoch loss history (epochLossHistory) and learning rate history (lrHistory), but only displays them as text lists.

Proposed enhancement:

• Add a line chart showing loss over epochs
• Optionally overlay learning rate on a secondary axis
• Show convergence visually — this would reinforce the learning concepts the app teaches

Recharts is already in the dependency tree (via shadcn/ui charts), so no new dependencies needed.