feat: SDK hardening integration (sign-callback recovery + apply-after-submit)

[WiktorStarczewski]

Summary

Thinned-down replacement for #189. Contains only the SDK-hardening improvements that are backed by external SDK PRs (miden-client / web-sdk) — the MidenClient singleton unification is dropped entirely, and the client singleton is left untouched.

Supersedes #189.

What's included

1. Typed sign-callback failure recovery — when the wallet gets locked mid-transaction the sign callback throws; the failure is recovered via the SDK's lastAuthError() and the tx is left Queued for retry after unlock instead of marked Failed (which previously caused note loss).
   • SDK: feat(web): expose lastAuthError() for typed sign-callback failure recovery miden-client#2058
2. ApplyTransactionAfterSubmitFailed handling — a tx that submits on-chain but fails the local apply step is marked Completed (not Failed) via the SDK's errorCode dispatch, so the user sees "Transaction sent" rather than a confusing failure for a tx that IS on chain. InputNoteAlreadyConsumedOnChain is cancelled.
   • SDK: feat(client): distinct ApplyTransactionAfterSubmitFailed error + one retry miden-client#2059, feat(web): expose stable errorCode on thrown JS errors for variant dispatch miden-client#2060
3. Transaction-stage progress UI — a stage field (syncing / sending / confirming / delivering) drives per-stage labels in the progress modal during the spinner window.
4. Stress-suite transport-failure perturbation (STRESS_TRANSPORT_FAIL_PROB) — end-to-end coverage of the SDK's durable relay outbox (fix(rust): durable NTL relay for Client::send_private_note miden-client#2127).

The per-tx sign callback is wired through the existing getMidenClient(options) path; the error classification (buildSignCallbackError + lastAuthError() recovery) all degrades gracefully if the SDK methods aren't present.

What's dropped vs #189

• The entire MidenClient singleton unification — keystore-bridge.ts, keystore-wiring.ts, Vault.encryptKeystoreEntry(), the two-instance→one-singleton collapse, the getMidenClient() signature change, and the adapter/main.ts wiring. miden-client.ts, vault.ts, miden-client-interface.ts, and the intercom adapters are unchanged from base.
• Item 3 (waitForIdle() in Actions.lock()), which was backed by the draft/exploratory miden-client#2057 — switching lock() to a runtime-guarded waitForIdle() would no-op (regressing the existing withWasmClientLock drain) until #2057 lands, so lock() is left as-is.

Verification

• Full unit suite: 1662/1662 pass
• tsc --noEmit: clean
• eslint: clean on changed files

Diff: 26 files, +1216 / −70 (vs #189's 44 files, +1867 / −577).

Note: like #189, this branch is based on the pre-#189 merge-base and is stale vs current main (~52 commits) — rebasing onto main is a separate step.