Public link sharing & invite-link auto-signup

Generate a public read-only link for a resource (especially budgets) — anyone with the link can view it without an account. Optional “virtual transactions” on shared budgets so link visitors can contribute their own spending to the budget without affecting your stats.

Also: when you invite someone whose email isn’t registered yet, they get a real sign-up email and a flow that takes them straight to the invitation after sign-up (today, invitations to unregistered emails go nowhere).

Note: partly overlaps with the existing “Virtual Expenses for Shared Budgets” request — virtual transactions on link-shared budgets is one bullet of this larger sub-feature.

Part of: https://moneymatter.featurebase.app/p/household-sharing-umbrella