chore: adiciona robots.txt bloqueando rotas privadas do app

Permite indexação apenas da landing page e do llms.txt. Bloqueia todas
as rotas autenticadas (dashboard, transações, contas, cartões, etc.)
e a API.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

public/robots.txt

@@ -0,0 +1,20 @@
+User-agent: *
+Allow: /$
+Allow: /llms.txt
+Disallow: /dashboard
+Disallow: /transactions
+Disallow: /accounts
+Disallow: /cards
+Disallow: /categories
+Disallow: /budgets
+Disallow: /payers
+Disallow: /notes
+Disallow: /insights
+Disallow: /calendar
+Disallow: /inbox
+Disallow: /attachments
+Disallow: /reports
+Disallow: /settings
+Disallow: /login
+Disallow: /signup
+Disallow: /api/