feat(desktop): multi-transport hosts with relay fallback, card-style services dropdown

- A saved host now keeps every transport its pairing link carried: direct URL
  plus the relay descriptor, with one token for both (the mobile connection
  model). Switching tries the direct leg and falls back to the E2EE tunnel;
  list probes report Connected · Relay when only the tunnel reaches the host;
  relaunch restore picks direct first
- Host switching trusts the dropdown's fresh probe instead of re-probing on
  click (no doubled latency, no transient Unreachable flashes); statuses are
  written once with the final outcome, survive the dropdown closing via a
  last-known cache, and an unprobed host reads Checking — never Unknown
- Open-in-new-window works for relay hosts: a new IPC command boots the local
  UI with the host id injected and the renderer picks the transport; the app
  render holds on the relay restore so the splash shows instead of a transient
  auth screen (10s safety valve)
- Relay host control socket gained protocol-level keepalive: a missed pong
  window terminates and reconnects, so the relay can no longer hold a ghost
  registration that leaves every client tunnel hanging; the desktop relay
  probe also hard-times-out at 8s instead of hanging status flows
- Services dropdown restyled with mobile-style cards: per-provider usage
  cards, per-host instance cards with a selected highlight and a toned
  status line, MCP servers grouped in a card

packages/electron/preload.mjs

@@ -62,6 +62,14 @@ if (clientToken && isLocalPage) {
   contextBridge.exposeInMainWorld('__OPENCHAMBER_CLIENT_TOKEN__', clientToken);
 }
 
+// Which saved host this window should connect to over the relay-capable path
+// (direct probe first, E2EE tunnel fallback). Local pages only — the id is
+// only useful together with the desktop IPC channel anyway.
+const relayHostId = readArgValue('--openchamber-relay-host-id');
+if (relayHostId && isLocalPage) {
+  contextBridge.exposeInMainWorld('__OPENCHAMBER_RELAY_HOST_ID__', relayHostId);
+}
+
 if (runtimeHeadersRaw && isLocalPage) {
   try {
     const runtimeHeaders = JSON.parse(runtimeHeadersRaw);