fix(ui): post-Trail review — alert visible everywhere + writeTo error checks

Four findings from a code review pass on the Trail feature:

- showAlert overlay was gated to `curr == home`, so feedback like
  "Tracking started", "Trail saved", "GPX N B (USB)" never rendered on
  the Trail screen (or QuickMsg / Settings / Auto-Advert, for that
  matter). Drop the gate — the overlay shows on whichever screen the
  caller invoked from.
- TrailStore::writeTo only verified the first write; subsequent point
  writes could partially fail (full filesystem, mid-write power loss)
  and the method would still return true. Check every write return
  value so handleSave's "Trail saved" only fires on actual success.
- gpxPoint: clamp snprintf's intended-length return against the buffer
  size and skip the point if gmtime returns null. Avoids any chance of
  out.write reading past buf[120].
- TrailScreen::_act_map sized exactly to today's 8 actions — bump to 12
  so adding another popup item later doesn't cause an OOB write into
  the surrounding members.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

examples/companion_radio/ui-new/UITask.cpp

@@ -1695,7 +1695,7 @@ void UITask::loop() {
     } else if (!_locked && millis() >= _next_refresh && curr) {
       _display->startFrame();
       int delay_millis = curr->render(*_display);
-      if (millis() < _alert_expiry && curr == home) {  // render alert only on home screen
+      if (millis() < _alert_expiry) {  // alert overlay on top of any screen
         _display->setTextSize(1);
         int lh    = _display->getLineHeight();
         int pad   = 3;