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>

src/helpers/ui/UIScreen.h

@@ -57,5 +57,10 @@ public:
   virtual int render(DisplayDriver& display) =0;   // return value is number of millis until next render
   virtual bool handleInput(char c) { return false; }
   virtual void poll() { }
+  // Called by UITask::setCurrScreen() each time this screen becomes current —
+  // the place to reset per-visit state (selection, dirty flag, sub-views).
+  // Default no-op for screens that keep state across visits. Because it's
+  // invoked centrally, a new screen can't "forget" to be reset on show.
+  virtual void onShow() { }
 };