feat(ui): trail Units setting — km/h, mph, min/km, min/mi

Second row in the Config view: UP/DOWN moves focus between Min dist
and Units, LEFT/RIGHT cycles the focused row's value. Units propagates
to the Summary view, which now shows either "Avg: N km/h" / "N mph"
or "Pace: M:SS /km" / "/mi" depending on the choice.

Pace renders as "--:-- /unit" when distance or elapsed time is zero
(avoids divide-by-zero glitches with one-point trails). mph conversion
uses the 0.621371 ratio with rounding; pace works in floating point
off raw metres and seconds for accuracy at low speeds.

NodePrefs gains trail_units_idx as another reserved-ish byte; old saves
load with 0 = km/h, no schema bump needed since the rd() lambda already
defaults missing fields to zero.

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

examples/companion_radio/NodePrefs.h

@@ -103,6 +103,7 @@ struct NodePrefs {  // persisted to file
   // not a persisted preference.
   uint8_t trail_interval_idx;   // reserved — sampling cadence is now fixed at TrailStore::SAMPLING_SECS
   uint8_t trail_min_delta_idx;  // indexes TrailStore::minDeltaMeters (5/10/25/100 m)
+  uint8_t trail_units_idx;      // indexes TrailStore::unitLabel (km/h, mph, min/km, min/mi)
 
   // Tail sentinel written at the end of /new_prefs. Bump the low byte when
   // adding/removing/reordering fields in DataStore::savePrefs/loadPrefsInt so