Add end-of-game “Session Summary” modal + save flow

After a game ends, show a summary modal (score, duration, key metrics) and save results to backend.
Modal appears reliably at end of a session.
Save request sent exactly once per session (no duplicates on refresh/back).
Clear success/failure UI (e.g., “Saved” vs retry option).
Manual test steps documented (or a component test if available).
