perf(l2): no redraws while nothing changes

Found by counting flushes on idle screens in the sim:
- The compose field's cursor blinked (a redraw every 400 ms) as soon as
  a conversation opened, the keyboard hidden: lv_keyboard_set_textarea()
  focuses the field, and LVGL only stops a blink when the cursor moves.
  fieldFocus() sets the state and nudges the blink; also used for the
  WiFi and channel fields, which left theirs running.
- lv_obj_align_to() restyles on every call, so the status bar's icons
  redrew every second; placeLeftOf() moves them only when they move.
- Text colours set every second (nav bar, GPS status, Diag errors) go
  through setTextColor(), which skips an unchanged colour.
- A LV_LABEL_LONG_DOT label writes its "..." into its own text, so
  setText() never matched and rewrote it every refresh (the nav bar's
  hint); labelShows() reads past the dots.
- The map area, the area-outline, trail and vector-label layers were
  invalidated on every map refresh (every 3 s, a full-map redraw); now
  only when the view or what they draw changed.

Idle map: 17 -> 1 flush in 5 s; list and settings screens: 0.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

examples/companion_radio/ui-lvgl/map/VectorLabels.h

@@ -118,8 +118,12 @@ static void clear() {
 // Places the labels for a view (world px of its top-left at zoom z, size;
 // `bottom` px taken by a bar along the bottom).
 static void layout(double left, double top, int w, int h, int z, bool on, int bottom) {
+  int had = s_n;
   clear();
-  if (!on || z < 10 || !s_layer || !s_placed || !s_cand) { if (s_layer) lv_obj_invalidate(s_layer); return; }
+  if (!on || z < 10 || !s_layer || !s_placed || !s_cand) {   // none: those shown go (none shown, nothing to redraw)
+    if (s_layer && had) lv_obj_invalidate(s_layer);
+    return;
+  }
   int dz = z >= 14 ? 14 : z >= 12 ? 12 : 10, k = z - dz;
   double span = (double)(TILE_PX << k);   // a data tile, in px at zoom z
   int x0 = (int)floor(left / span), x1 = (int)floor((left + w) / span);