feat(units): global metric/imperial setting for all distance displays

Add a single "Units" choice (Settings › System: Metric / Imperial) that
governs every distance and speed shown in Tools — Nearby Nodes, the Trail
views, and the navigate-to-point screen.

- NodePrefs: units_imperial (global) + trail_show_pace (Trail readout mode),
  schema bumped 0xC0DE0005 → 0006. The old combined trail_units_idx is kept
  for file-layout stability but no longer drives anything; upgraders default
  to metric + speed.
- geo::fmtDist gains an imperial mode (ft below ~1000 ft, then miles).
- NavView / Nearby / Trail (waypoint list, Summary distance, List deltas,
  map scale-bar label) all format through the global preference.
- Trail's action-menu "Units" row becomes a Speed/Pace readout toggle; the
  km-vs-mi unit now follows the global setting instead of being one of four
  combined options.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

examples/companion_radio/ui-new/SettingsScreen.h

@@ -56,6 +56,7 @@ class SettingsScreen : public UIScreen {
     SECTION_SYSTEM,
     TIMEZONE,
     LOW_BAT,
+    UNITS,
     // Contacts section
     SECTION_CONTACTS, DM_FILTER, CH_FILTER, ROOM_FILTER,
     // Messages section
@@ -486,6 +487,10 @@ class SettingsScreen : public UIScreen {
       display.print("LowBat");
       display.setCursor(display.valCol(), y);
       display.print(LOW_BAT_LABELS[lowBatIndex()]);
+    } else if (item == UNITS) {
+      display.print("Units");
+      display.setCursor(display.valCol(), y);
+      display.print((p && p->units_imperial) ? "Imperial" : "Metric");
     } else if (item == BATT_DISPLAY) {
       display.print("BattDisp");
       display.setCursor(display.valCol(), y);
@@ -732,6 +737,11 @@ public:
       if (left)  idx = (idx + LOW_BAT_COUNT - 1) % LOW_BAT_COUNT;
       if (left || right) { p->low_batt_mv = LOW_BAT_OPTS[idx]; _dirty = true; return true; }
     }
+    if (_selected == UNITS && p && (left || right || enter)) {
+      p->units_imperial ^= 1;
+      _dirty = true;
+      return true;
+    }
     if (_selected == BATT_DISPLAY && p) {
       int idx = p->batt_display_mode < BATT_DISPLAY_COUNT ? p->batt_display_mode : 0;
       if (right) idx = (idx + 1) % BATT_DISPLAY_COUNT;