Remove stale hosted free copy

[jaeyunha]

Summary

• Replace stale pricing hero copy that still said "Start free"
• Replace billing empty-state copy that still told operators to seed the Free plan
• Strengthen docs/free-tier regression test to cover these strings

Verification

• bunx vitest run tests/docs-no-free-tier.test.ts
• make check
• push guardrails: typecheck + Biome passed

The production page at https://opensend.namuh.co/pricing is currently serving the old deployed copy until this hotfix is merged and deployed.