feat: add forecast desk route with trend snapshots

[iamasx]

Summary

• add the /forecast-desk route with a comparison band, trend snapshot cards, and short forecast notes
• keep the route self-contained with local mock data and responsive layout primitives already used in the app
• add route tests covering hero actions, comparison metrics, snapshots, and forecast notes

Closes #214

Verification

• npx eslint src/app/forecast-desk/page.tsx src/app/forecast-desk/page.test.tsx src/app/forecast-desk/_data/forecast-desk-data.ts
• npx tsc --noEmit
• npx vitest run src/app/forecast-desk/page.test.tsx (blocked locally on Node v20.15.0 because the checked-in vitest.config.ts and the installed jsdom dependency chain both require a newer ESM-compatible runtime)