feat(remote-connect): account-password mobile pairing and resilient device list

[bobleer]

Summary

When the desktop is logged into a BitFun account, QR pairing now requires the same account's username+password instead of a free-form user ID, and the mobile device list transparently recovers from expired delegated identities instead of showing a terminal 'authorization expired' state.

Design spec: docs/superpowers/specs/2026-07-20-mobile-web-account-pairing-and-device-refresh-design.md

Pairing (account mode)

• QR advertises auth=account with username prefill when the desktop is logged in; unsigned-in desktops keep the legacy user-ID flow.
• Mobile submits username+password through the ECDH-encrypted room channel; the password is never persisted or logged, and password-less auto-reconnect is disabled in account mode.
• Desktop verification is verify-only: challenge fetch + local KEK unwrap + comparison against the current session's master key. /api/auth/login is never called, so the desktop token is not refreshed or polluted, and the credentials must match the account currently signed in on the scanned desktop.
• On success the trusted mobile identity binds the canonical account user id (not the plaintext username), so reconnects keep matching.
• Account credentials are never downgraded: if the verifier is gone (desktop signed out after generating the QR), a pairing request carrying a password is rejected with a clear re-login message instead of silently falling back to password-less pairing.
• Login, logout, and account switches refresh or clear the pairing context and the URL-bound trusted identity; the real verification failure cause stays in desktop logs while the mobile receives a unified rejection message (no account enumeration).

Device list resilience

• RelayHttpClient: clearDelegatedIdentity(), requestDelegatedIdentity({ force }), and a shared 401 handler that clears the delegated identity, force-refreshes it from the paired desktop, and retries listDevices / sendDeviceRpc once.
• DevicesPage: the terminal tokenExpired state and its i18n keys are removed; failures fall back to retryable errors or the noDelegatedIdentity state.

Verification

• cargo test -p bitfun-core --lib remote_connect::tests — 5 passed (new focused tests: missing credentials, no-downgrade guard, canonical id binding)
• cargo test -p bitfun-services-integrations --features remote-connect --test remote_connect_contracts — 51 passed
• cargo check -p bitfun-desktop — clean
• pnpm --dir src/mobile-web run type-check — clean