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/web/server/lib/opencode/bootstrap-runtime.js

@@ -26,6 +26,8 @@ export const createBootstrapRuntime = (dependencies) => {
       getRelayPairingCandidate,
       reconcileRelay,
       getPairingTransports,
+      getDirectCandidateUrls,
+      getServerId,
       getServerLabel,
       readSettingsFromDiskMigrated,
       normalizeTunnelSessionTtlMs,
@@ -76,6 +78,7 @@ export const createBootstrapRuntime = (dependencies) => {
       serverStartedAt,
       gracefulShutdown,
       getHealthSnapshot,
+      getServerId,
       tunnelAuthController,
       uiAuthController,
     });
@@ -91,6 +94,8 @@ export const createBootstrapRuntime = (dependencies) => {
       getRelayPairingCandidate,
       reconcileRelay,
       getPairingTransports,
+      getDirectCandidateUrls,
+      getServerId,
       getServerLabel,
       readSettingsFromDiskMigrated,
       normalizeTunnelSessionTtlMs,