fix(stats): hide zero-usage countries

[opencode-agent]

Summary

• render countries with zero token usage as white/transparent on the world map
• preserve full opacity for active countries with nonzero usage
• apply the behavior to both the data homepage and model detail maps

Checks

• bun typecheck (packages/stats/app)
• bun --bun vite build (packages/stats/app)
• bun x prettier --check src/routes/index.tsx "src/routes/[lab]/[model].tsx" src/routes/index.css

Requested by: @fwang (Frank via Slack)