fix: ping RTT 0ms, degree symbol, clock page dots overlap

- Ping: add _ping_ack_seen flag; RTT is measured only after ACK entry
  was first confirmed present in the table, preventing false 0ms when
  isAckPending() returns false before the entry is registered
- NearbyScreen: replace unsupported degree symbol \xb0 with 'd'
  in bearing format ("Dist:1.5km 45d")
- Clock page: hide page indicator dots (already hides node name/battery);
  eliminates overlap with size-2 clock text drawn at y=0

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

examples/companion_radio/ui-new/UITask.cpp

@@ -1803,8 +1803,8 @@ public:
     // ensure current page is visible (e.g. after settings change)
     if (!isPageVisible(_page)) _page = navPage(_page, +1);
 
-    // curr page indicator — only visible pages get a dot
-    {
+    // curr page indicator — hidden on CLOCK page (full screen used for dashboard)
+    if (_page != CLOCK) {
       int vis_count = 0, curr_vis = 0;
       for (int i = 0; i < (int)Count; i++) {
         if (!isPageVisible(i)) continue;