feat(eink): suppress clock seconds to prevent per-second panel refreshes

E-ink panels take ~2 s for a full refresh and visible flicker on every update.
Showing seconds caused the clock page to redraw every 1 s, triggering a real
panel refresh each time the CRC changed.

- Default clock_hide_seconds=1 for EINK_DISPLAY_MODEL builds so fresh
  installs don't flicker out of the box.
- Remove the Seconds setting from SettingsScreen on e-ink so users can't
  accidentally re-enable it.
- Clock page now returns 60 s refresh interval on e-ink (content changes at
  most once per minute); other home pages capped at 30 s (new messages still
  force immediate redraw via notify()).
- Lock screen refresh interval raised from 1 s to 60 s on e-ink; button
  presses already reset _next_refresh=0, so unlock feedback is instant.

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

examples/companion_radio/MyMesh.cpp

@@ -952,6 +952,9 @@ MyMesh::MyMesh(mesh::Radio &radio, mesh::RNG &rng, mesh::RTCClock &rtc, SimpleMe
   _prefs.dm_show_all = 1;        // show all contacts by default
   memset(_prefs.dm_notif, 0, sizeof(_prefs.dm_notif));
   _prefs.auto_off_secs = 15;    // 15 seconds auto-off by default
+#ifdef EINK_DISPLAY_MODEL
+  _prefs.clock_hide_seconds = 1;  // e-ink: seconds cause a panel refresh every second
+#endif
   _prefs.tz_offset_hours = 0;  // UTC by default
   _prefs.low_batt_mv = 3400;  // auto-shutdown at 3.4V by default
   _prefs.batt_display_mode = 0; // icon by default