Expose workspace invitation links

[chengweiv5]

Summary

• Return invite_url on workspace invitation API responses.
• Parse invitation responses through the core API schema with an older-backend fallback.
• Show pending invitation links and copy actions in Settings → Members, including the invite-created toast.

Verification

• GOTOOLCHAIN/GOPATH/GOCACHE under /tmp go test ./internal/handler -run TestCreateInvitation
• corepack pnpm exec vitest run packages/core/api/schemas.test.ts
• pnpm typecheck (with a temporary /tmp pnpm shim for Turbo package-manager lookup)