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/DiagScreen.h

@@ -99,7 +99,7 @@ static void setLive(int i, const diag::Row& r) {
   char v[40];
   showValue(r.label, r.value, v, sizeof(v));
   setText(lv.val, v);
-  if (!strcmp(r.label, "Errors")) lv_obj_set_style_text_color(lv.val, lv_color_hex(strcmp(r.value, "OK") ? theme::FAIL : theme::OK), 0);
+  if (!strcmp(r.label, "Errors")) setTextColor(lv.val, strcmp(r.value, "OK") ? theme::FAIL : theme::OK);
 }
 
 // A packets row: the name, then received and sent in fixed columns.