multisig-client: move `nonce` into the options object of proposal-creation APIs

Context

createP2idProposal(recipient, faucet, amount, nonce?, options?) places the optional
nonce positionally before the options object, so callers who only want options must
pass a positional undefined:

await multisig.createP2idProposal(
  recipientAccountId,
  faucetAccountId,
  1000n,
  undefined,                    // nonce hole
  { reclaimHeight: 500_000 },
);

Raised during review of #381 (comment):
the undefined hole makes every documented example ugly and the call sites error-prone.
The same shape applies to the rest of the create*Proposal family — options added by
future features (as #322 added noteType and #366 added reclaimHeight/timelockHeight)
make the positional nonce progressively worse.

Proposal

Fold nonce into the options object so the 4th parameter is a single options bag:

await multisig.createP2idProposal(recipientAccountId, faucetAccountId, 1000n, {
  nonce: 42,                    // optional, defaults to Date.now() as today
  noteType: NoteType.Private,
  reclaimHeight: 500_000,
});

Two migration options:

1. Deprecation window — detect the 4th parameter's type (number ⇒ legacy nonce,
   object ⇒ options bag) and keep both working for a release, with the positional form
   marked @deprecated.
2. Clean break — change the signature outright in the next breaking release. If the
   #366 release ships as 0.17.0 (breaking on the Rust side anyway), this change could
   ride the same window and skip the type-detection dance entirely.

The wire format is unaffected either way — this is purely call-signature ergonomics.

Scope

• createP2idProposal, createConsumeNotesProposal, createAddSignerProposal,
  createRemoveSignerProposal, createChangeThresholdProposal,
  createSwitchGuardianProposal, createUpdateProcedureThresholdProposal
• Example wrappers: examples/_shared/multisig-browser, examples/web, and the
  smoke-web harness
• Docs: docs/MULTISIG_SDK.md, packages/miden-multisig-client/README.md