feat: handle registration screen and HTTP resolution polling (MM-148) #3

Summary#

Adds register_handle Rust IPC command: fetches relay domain from describeServer, reads DID + session token from Keychain, POSTs to POST /v1/handles
Adds check_handle_resolution Rust IPC command: polls resolveHandle endpoint, never rejects (safe for setInterval)
Adds HandleRegistrationScreen.svelte: auto-runs on mount, shows polling progress after registration, 2-minute timeout with graceful proceed
Wires handle_registration step into the onboarding state machine between shamir_backup and complete

Complete onboarding through Shamir backup; confirm handle_registration step appears next
Verify handle badge shows alice.your-domain.com during polling phase
With relay running: confirm registration succeeds and onsuccess fires on resolution
With relay returning 409 HANDLE_TAKEN: confirm error message and retry button appear
With DNS propagation slow: confirm ontimeout fires after 2 minutes and onboarding continues
Rust serialization tests: cargo test -p identity-wallet register_handle (8 tests pass)