Create prediction dashboard component

[romahawk]

Part of #59

What

Build a summary view that shows velocity trends and completion forecasts for all active projects.

Tasks

• Create components/ai/PredictionDashboard.tsx
• Render velocity sparkline per project (last 4 weeks)
• Show completion probability, deadline, and forecast completion date
• Add to Projects Module as a collapsible section

AC

• Dashboard renders for 1–5 active projects without layout breaking
• Sparklines render correctly with ≥ 2 data points; show placeholder for < 2