[fix](trx-frontend-http): don't redirect to login on 403 (insufficient permissions)

Only redirect to login on 401 (unauthenticated). For 403 errors
(authenticated but insufficient role), let the caller handle the error.

This prevents rx-authenticated users from being redirected to login
when they attempt to scroll the jog wheel or frequency input, which
tries to call /set_freq (a control-only endpoint).

RX users will now see "Insufficient permissions" hint instead of
being sent to login screen.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Stanislaw Grams <stanislawgrams@gmail.com>

src/trx-client/trx-frontend/trx-frontend-http/assets/web/app.js

@@ -961,13 +961,17 @@ function disconnect() {
 
 async function postPath(path) {
   const resp = await fetch(path, { method: "POST" });
-  if (resp.status === 401 || resp.status === 403) {
-    // Auth error - return to login
+  if (resp.status === 401) {
+    // Not authenticated - return to login
     authRole = null;
     if (es) es.close();
     showAuthGate();
     throw new Error("Authentication required");
   }
+  if (resp.status === 403) {
+    // Authenticated but insufficient permissions - don't redirect
+    throw new Error("Insufficient permissions");
+  }
   if (!resp.ok) {
     const text = await resp.text();
     throw new Error(text || resp.statusText);