fix(ui): late Admin login reply could be misrouted after giving up

UITask::onRoomLoginResult() dispatches a login reply to whichever
screen is *currently* shown (curr == admin_screen ? AdminScreen :
MessagesScreen), not to whoever actually sent the request. Neither
giving up path (manual Cancel, or the timeout added in 23f43cac) told
MyMesh to stop tracking the request, so a reply that still arrived
after the user had navigated away landed on whatever screen they'd
moved to instead -- most likely MessagesScreen, which then persisted
its own unrelated _login_pw as the "confirmed" password for that
pubkey, silently corrupting the saved password even on a genuine
success.

Adds MyMesh::cancelUiPendingLogin(pub_key), pubkey-guarded so it's a
no-op if a newer request has since overwritten ui_pending_login, called
from both of AdminScreen's give-up paths. A late reply now simply
matches nothing and is dropped.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

examples/companion_radio/ui-new/AdminScreen.h

@@ -436,6 +436,11 @@ public:
   void poll() override {
     if (_phase == LOGIN && _login_waiting && (int32_t)(millis() - _login_deadline_ms) >= 0) {
       _login_waiting = false;
+      // Stop tracking this request on the MyMesh side too -- otherwise a reply
+      // that still arrives after we've given up gets misrouted to whatever
+      // screen the user is on by then (see cancelUiPendingLogin()'s comment),
+      // corrupting that screen's unrelated login state with our answer.
+      the_mesh.cancelUiPendingLogin(_target.id.pub_key);
       // No response at all is ambiguous (could be a wrong/stale password, could
       // just be out of range) -- but a saved password that's gone stale (the
       // remote's password changed) is exactly this: silence, not a nack. Treat
@@ -515,7 +520,11 @@ public:
   bool handleInput(char c) override {
     if (_phase == LOGIN) {
       if (_login_waiting) {
-        if (c == KEY_CANCEL) { _login_waiting = false; returnToOrigin(); }
+        if (c == KEY_CANCEL) {
+          _login_waiting = false;
+          the_mesh.cancelUiPendingLogin(_target.id.pub_key);   // see cancelUiPendingLogin()'s comment
+          returnToOrigin();
+        }
         return true;
       }
       auto r = kb().handleInput(c);