Unify Template type definitions across frontend and backend

[GeneralJerel]

Problem

There are three separate Template type definitions that can drift out of sync:

• UITemplate in apps/agent/src/templates.py (backend)
• Template interface in apps/app/src/components/template-library/index.tsx (frontend)
• SeedTemplate in apps/app/src/components/template-library/seed-templates.ts (frontend)

SeedTemplate is missing component_type and component_data fields, so seed templates for charts would need to be cast. Any field added to one type but not the others causes silent mismatches.

Suggested fix

• Unify Template and SeedTemplate into a single shared frontend type
• Ensure the frontend type matches the backend UITemplate schema (all optional fields present)
• Consider a shared schema definition (e.g. Zod on frontend, matching TypedDict on backend)

Files

• apps/agent/src/templates.py
• apps/app/src/components/template-library/index.tsx
• apps/app/src/components/template-library/seed-templates.ts

From PR #20 review