refactor(companion): hoist screen entry into virtual UIScreen::onShow()

Replace the ad-hoc enter()/markClean() entry methods (which lived outside
the UIScreen interface and were invoked via casts from each gotoX) with a
virtual onShow() lifecycle hook called centrally by setCurrScreen().

This removes the "forgot to call enter() in a new navigator" footgun and
the unchecked cast smell: 12 navigators collapse to one-line
setCurrScreen(x) calls, and override enforces signature match. Two entries
that carry a parameter/variant keep an explicit typed call after
setCurrScreen(): RingtoneEditor::selectSlot(slot) and TrailScreen::showMapView().

Behaviour-preserving: only screens that previously had enter() get an
onShow() override; Splash/Home/QuickMsg/Diag keep no reset as before.

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

examples/companion_radio/ui-new/RepeaterScreen.h

@@ -128,7 +128,7 @@ class RepeaterScreen : public UIScreen {
 public:
   RepeaterScreen(UITask* task) : _task(task), _dirty(false), _sel(0), _scroll(0), _item_count(1) {}
 
-  void enter() {
+  void onShow() override {
     _dirty = false; _sel = 0; _scroll = 0;
     _picker.menu.active = false; _editor.freq.active = false;
     _picker.saving = false; _picker.deleting = false;