feat: connection candidates refresh + relay identity hardening

Candidates refresh (server + mobile + desktop clients):
- GET /api/client-auth/connection/candidates returns the server's current
  LAN URLs, relay candidate, and serverId for already-paired devices
- /health and /api/version expose serverId so clients can verify a learned
  address belongs to the expected server before sending their bearer token
- mobile: refresh saved candidates over the live transport after every
  connect/wake, hot-switch relay->LAN when a fresh address is reachable;
  serverId gate on direct probes; token no longer sent to /health
- desktop: refresh stored host apiUrl after a relay connect and hot-switch
  back to direct; electron probe verifies serverId before authenticated fetch

Fixes found while debugging a dead pairing:
- settings: strict reader that throws on corrupt/unreadable file instead of
  returning {}; relay signing/encryption key generation is now gated on it,
  so a swallowed read failure can no longer mint a new server identity and
  orphan every paired device (loud log when a keypair IS generated)
- SessionAuthGate: bounded auto-retry for transient session-check failures
  (initial request racing the relay tunnel's first WS attempt, startup 5xx)

packages/ui/src/components/desktop/DesktopHostSwitcher.tsx

@@ -31,6 +31,7 @@ import {
   type DesktopHost,
   type HostProbeResult,
 } from '@/lib/desktopHosts';
+import { scheduleDesktopHostCandidateRefresh } from '@/lib/desktopRelayRestore';
 import { getRuntimeApiBaseUrl, getRuntimeKey, subscribeRuntimeEndpointChanged, switchRuntimeEndpoint } from '@/lib/runtime-switch';
 import {
   desktopSshConnect,
@@ -519,6 +520,9 @@ export function DesktopHostSwitcherDialog({
         runtimeKey: runtimeKeyForHost(host),
         relay,
       });
+      // On the relay: learn the server's current LAN address in the background
+      // and hot-switch back to direct if the stored one merely went stale.
+      scheduleDesktopHostCandidateRefresh(host.id);
     };
 
     const origin = host.id === LOCAL_HOST_ID ? localOrigin : (normalizeHostUrl(host.url) || '');