fix(funnel): preserve selected credit pack through signup

[cursor]

Summary

• Preserve the clicked credit pack when anonymous pricing checkout redirects through signup/login.
• Allow only exact /pricing?pack=starter|pro|business signup return targets, keeping other query strings rejected.
• Add focused intent-funnel regression coverage for the allowlist and pricing auth redirects.

Validation

• npm run build
• node tests/intent-funnel.test.mjs