fix(dashboard): auto-reload SPA on stale-token 401 in loopback mode (#33861)

The dashboard's loopback auth uses an ephemeral '_SESSION_TOKEN' that
rotates on every server restart (hermes update, hermes gateway restart,
etc.). A tab kept open across the restart holds the OLD token in
window.__HERMES_SESSION_TOKEN__ from the previous HTML render, so every
'/api/*' fetch returns '401 Unauthorized' — surfacing in the UI as
'Failed to load Kanban board: 401: Unauthorized', 'Analytics 401', etc.
(#24186, #25275).

Before this patch the workaround was to manually clear site data or
hard-reload — annoying enough that users reported it as a regression
even though the token rotation is by design (security property:
stolen tokens can't survive a server restart).

The HTML response already sets 'Cache-Control: no-store, no-cache,
must-revalidate', so a reload reliably picks up the freshly-injected
token. fetchJSON now triggers that reload automatically on the first
loopback-mode 401, guarded by a sessionStorage flag so a genuine
auth bug (where even the new token fails) falls through to throw
on the second attempt instead of reload-looping. The flag is
cleared on any 2xx so a subsequent server restart in the same tab
gets its own reload cycle.

Gated mode is unaffected — that path already redirects to login_url
via the structured 401 envelope (Phase 6), and the new code is
explicitly skipped when window.__HERMES_AUTH_REQUIRED__ is set.

Refs #24186, #25275

web/src/lib/api.ts

@@ -91,6 +91,43 @@ export async function fetchJSON<T>(url: string, init?: RequestInit): Promise<T>
       // Never resolve — the page is about to unload.
       return new Promise<T>(() => {});
     }
+    // Loopback mode: ``_SESSION_TOKEN`` rotates on every server restart
+    // (``hermes update``, ``hermes gateway restart``, etc.). A tab kept
+    // open across the restart holds the OLD token in
+    // ``window.__HERMES_SESSION_TOKEN__`` from the previous HTML render,
+    // so every fetch returns 401. The HTML is served ``Cache-Control:
+    // no-store`` so a reload picks up the freshly-injected token. Trigger
+    // that reload once on the first stale-token 401 — gated mode is
+    // handled above, so reaching here in gated mode means a real
+    // middleware failure that should not reload-loop.
+    if (!window.__HERMES_AUTH_REQUIRED__) {
+      let alreadyReloaded = false;
+      try {
+        alreadyReloaded =
+          sessionStorage.getItem("hermes.tokenReloadAttempted") === "1";
+      } catch {
+        /* SSR / privacy mode — fall through to throw */
+      }
+      if (!alreadyReloaded) {
+        try {
+          sessionStorage.setItem("hermes.tokenReloadAttempted", "1");
+        } catch {
+          /* SSR / privacy mode — best effort */
+        }
+        window.location.reload();
+        return new Promise<T>(() => {});
+      }
+    }
+  }
+  if (res.ok) {
+    // Clear the stale-token reload guard: a successful 2xx proves the
+    // current ``window.__HERMES_SESSION_TOKEN__`` is valid, so the next
+    // 401 — if any — should be allowed to trigger its own reload cycle.
+    try {
+      sessionStorage.removeItem("hermes.tokenReloadAttempted");
+    } catch {
+      /* SSR / privacy mode — ignore */
+    }
   }
   if (!res.ok) {
     const text = await res.text().catch(() => res.statusText);