refactor(ui): audit-pass fixes + dead-code/redundancy cleanup

Fixes from a full ui-new audit (OLED + e-ink both build green):
- remove dead DisplayDriver::drawScrollArrows (all lists use drawScrollIndicator)
- SettingsScreen selection bar -> lineStep()-1, matching the drawList screens
- HomeScreen: don't force the 1s blink refresh on the CLOCK page (status icons
  are hidden there anyway)
- FullscreenMsgView: clamp KEY_DOWN scroll to a cached _max_scroll instead of
  over-incrementing and leaning on the next render to clamp
- DataStore: clamp use_lemon_font (>1 -> 0) on load, like the other enum fields
- move the ~1.5KB wrap scratch (trans/lines) off the render stack into shared
  file-scope statics in FullscreenMsgView (single-threaded UI; the fullscreen
  view and history list never lay out in the same frame)

Cleanup:
- drop dead members RingtoneEditorScreen::DUR_VALS and HomeScreen::sensors_scroll
- delete redundant manual scroll-clamps in handleInput across the drawList
  screens (drawList already reclamps each render); remove the now-unused _visible
  from QuickMsgScreen/NearbyScreen/BotScreen and BotScreen::scrollToSel()

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

src/helpers/ui/DisplayDriver.h

@@ -162,16 +162,6 @@ public:
     }
   }
 
-  // Up/down scroll indicators in the right-edge column. top_y is the first
-  // row's y, bottom_y the last visible row's y. Replaces the 4-line
-  // setCursor/print("^")/setCursor/print("v") block in every scrollable list.
-  void drawScrollArrows(int top_y, int bottom_y, bool more_up, bool more_down) {
-    int x = width() - getCharWidth();
-    setColor(LIGHT);
-    if (more_up)   { setCursor(x, top_y);    print("^"); }
-    if (more_down) { setCursor(x, bottom_y); print("v"); }
-  }
-
   // Inverted title bar: light background, dark ellipsized label, then the
   // standard separator line. The label is UTF-8 translated by
   // drawTextEllipsized. Leaves ink colour LIGHT for following content.