chore(web): add per-file copyright headers to UI components

[Agbasimere]

Summary

• Prepend // Copyright (c) 2026 Sub Rosa contributors as the first line of Confetti.tsx, CountUp.tsx, and Toast.tsx so file-level ownership matches the root MIT LICENSE.
• Mechanical header-only change; no behavior, types, or exports were modified.

Test plan

• Confirm the copyright line is the first line of:
  • apps/web/src/ui/Confetti.tsx
  • apps/web/src/ui/CountUp.tsx
  • apps/web/src/ui/Toast.tsx
• CI: TypeScript coverage, contract tests, and bindings check are green
• Optional: pnpm web:typecheck and pnpm web:test

Closes: #211