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/core-routes.js

@@ -67,10 +67,29 @@ export const registerServerStatusRoutes = (app, dependencies) => {
     serverStartedAt,
     gracefulShutdown,
     getHealthSnapshot,
+    // Stable server identity (hash of the public signing key — not a secret).
+    // Exposed on /health and /api/version so a client can verify that a
+    // learned/probed address belongs to the expected server BEFORE sending its
+    // bearer token there. Optional: older wiring omits it.
+    getServerId = async () => null,
     tunnelAuthController = null,
     uiAuthController = null,
   } = dependencies;
 
+  // The identity is immutable for the process lifetime; resolve once, and never
+  // let an identity failure break health reporting.
+  let cachedServerId = null;
+  const resolveServerId = async () => {
+    if (cachedServerId) return cachedServerId;
+    try {
+      const value = await getServerId();
+      cachedServerId = typeof value === 'string' && value.trim() ? value.trim() : null;
+    } catch {
+      cachedServerId = null;
+    }
+    return cachedServerId;
+  };
+
   const allocateLoopbackPort = async () => {
     const net = await import('node:net');
     return await new Promise((resolve, reject) => {
@@ -213,24 +232,28 @@ export const registerServerStatusRoutes = (app, dependencies) => {
     }
   };
 
-  app.get('/health', (_req, res) => {
+  app.get('/health', async (_req, res) => {
+    const serverId = await resolveServerId();
     res.json({
       status: 'ok',
       timestamp: new Date().toISOString(),
       openchamberVersion,
       runtime: runtimeName,
       compatibility,
+      ...(serverId ? { serverId } : {}),
       ...getHealthSnapshot(),
     });
   });
 
-  app.get('/api/version', (_req, res) => {
+  app.get('/api/version', async (_req, res) => {
+    const serverId = await resolveServerId();
     res.json({
       status: 'ok',
       openchamberVersion,
       runtime: runtimeName,
       startedAt: serverStartedAt,
       compatibility,
+      ...(serverId ? { serverId } : {}),
     });
   });
 
@@ -371,6 +394,12 @@ export const registerAuthAndAccessRoutes = (app, dependencies) => {
     // server can actually be reached on (LAN derived from the server bind, not
     // the UI origin), for the create-device dialog.
     getPairingTransports = () => ({ local: null, lan: null, relayAvailable: true }),
+    // Returns ALL direct LAN URLs the server is currently reachable on (client-
+    // reached address first, then interface scan) for the candidates-refresh
+    // endpoint. Empty when the server is loopback-only.
+    getDirectCandidateUrls = () => [],
+    // Stable server identity for client-side verification of learned addresses.
+    getServerId = async () => null,
     // Display name a paired device shows for THIS server (issuing machine's
     // hostname), distinct from the per-device pairing label typed by the operator.
     getServerLabel = () => 'OpenChamber',
@@ -796,6 +825,48 @@ export const registerAuthAndAccessRoutes = (app, dependencies) => {
     });
   });
 
+  // Current reachable transports for an ALREADY-PAIRED device. Pairing-payload
+  // candidates are a snapshot: when DHCP hands this machine a new address, the
+  // device's saved LAN candidate goes stale and it is stuck on the relay forever.
+  // A client that connected over any live transport calls this to learn the
+  // server's present LAN URLs (plus the relay candidate when enabled) and update
+  // its saved candidate set. `serverId` lets the client bind the response — and
+  // later /health probes of the learned addresses — to this server's identity
+  // before trusting them with its bearer token.
+  // Auth: UI session or client bearer; never the short-lived URL token.
+  app.get('/api/client-auth/connection/candidates', async (req, res, next) => {
+    await runWithClientManagementAuth(req, res, next, async () => {
+      const candidates = [];
+      const directUrls = (() => {
+        try {
+          const urls = getDirectCandidateUrls(req);
+          return Array.isArray(urls) ? urls : [];
+        } catch {
+          return [];
+        }
+      })();
+      for (const url of directUrls) {
+        const normalized = normalizeCandidateUrl(url);
+        if (normalized) candidates.push({ type: 'lan', url: normalized, priority: 10 });
+      }
+      try {
+        const relayCandidate = await getRelayPairingCandidate({ ensureEnabled: false });
+        if (relayCandidate) candidates.push(relayCandidate);
+      } catch {
+        // Relay status failure must not break the direct-candidate refresh.
+      }
+      let serverId = null;
+      try {
+        const value = await getServerId();
+        serverId = typeof value === 'string' && value.trim() ? value.trim() : null;
+      } catch {
+        serverId = null;
+      }
+      res.setHeader('Cache-Control', 'no-store');
+      res.json({ label: getServerLabel(), ...(serverId ? { serverId } : {}), candidates });
+    });
+  });
+
   // Direct transports the server can be reached on (for the create-device dialog).
   app.get('/api/client-auth/pairing/transports', async (req, res, next) => {
     await runWithClientCreateAuth(req, res, next, async () => {