feat: relay URL configuration — runtime-configurable relay with Keychain persistence · pull #53

Summary#

Replace compile-time RELAY_CLIENT global static with runtime-configurable relay_client: OnceLock<RelayClient> on AppState; all four Tauri commands migrated to state-based access
Add get_relay_url / save_relay_url IPC commands with URL validation (normalize_relay_url), live health-check against /xrpc/_health, and Keychain persistence (store_relay_url / load_relay_url)
Add RelayConfigScreen.svelte onboarding step — pre-fills https://relay.ezpds.com, validates format, shows loading spinner and inline errors; returning users skip the screen on launch
Startup restore: run() reads relay URL from Keychain and initializes AppState before any command runs

cargo test — 6 new tests pass (normalize_relay_url_*, get_relay_url_returns_none_before_save, relay_url_round_trips_through_keychain); 2 pre-existing device_key sandbox failures unrelated to this change
cargo clippy -- -D warnings — clean
cargo fmt --all --check — clean
pnpm check and pnpm build — 0 errors
iOS Simulator smoke test — see docs/test-plans/2026-03-27-relay-url-config.md