Show member invite validation errors

[arjunkomath]

Summary

• return expected member-invite validation failures as action results instead of throwing
• show duplicate-member and invalid-invite details directly in the invite form toast

Why

Next hides thrown server action error messages in production and only exposes a digest, so expected business errors need to be returned as data.

Verification

• pnpm exec biome check actions/members.ts components/settings/member-settings.tsx
• pnpm exec tsc --noEmit

[github-actions]

React Doctor found no new issues. 🎉

_{Reviewed by React Doctor for commit 97d4638.}