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/lib/desktopHosts.ts

@@ -325,13 +325,16 @@ export const probeRelayDesktopHost = async (relay: DesktopHostRelay): Promise<Ho
   }
 };
 
-export const desktopHostProbe = async (url: string, options?: { clientToken?: string | null; requestHeaders?: Record<string, string> | null }): Promise<HostProbeResult> => {
+export const desktopHostProbe = async (url: string, options?: { clientToken?: string | null; requestHeaders?: Record<string, string> | null; expectedServerId?: string | null }): Promise<HostProbeResult> => {
   const invoke = getInvoke();
   if (!invoke) {
     return { status: 'unreachable', latencyMs: 0 };
   }
 
-  const raw = await invoke('desktop_host_probe', { url, clientToken: options?.clientToken || undefined, requestHeaders: options?.requestHeaders || undefined });
+  // `expectedServerId` makes the main-process probe verify the address's
+  // UNAUTHENTICATED /health identity before sending the bearer token — required
+  // when probing an address learned at runtime rather than typed by the user.
+  const raw = await invoke('desktop_host_probe', { url, clientToken: options?.clientToken || undefined, requestHeaders: options?.requestHeaders || undefined, expectedServerId: options?.expectedServerId || undefined });
   if (!isRecord(raw)) {
     return { status: 'unreachable', latencyMs: 0 };
   }