Duplicate modals stacking when openModal fires repeatedly

[Huygon764]

Problem:
The same modal can be opened multiple times and stack on top of itself:

• ModalLayout.openModal has no guard against duplicates — if two callers
  (double-click, race, effect re-run) call openModal("x") before the first
  instance closes, two instances of x are pushed into activeModals.
• useCommitmentGuard's effect re-runs while walletClient.account && !commitment && !isAuthenticated stays true, firing
  openModal("generateCommitment") repeatedly and producing overlapping
  generate-commitment modals.

Expectation:

• At most one modal instance per modal name is active at a time; repeated
  openModal(sameName) calls are no-ops while an instance is already open.
• Guard hooks fire openModal once per eligibility transition and reset after
  the user becomes ineligible (commitment created, logged in, or wallet
  disconnected).