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>

examples/companion_radio/ui-new/FullscreenMsgView.h

@@ -6,11 +6,22 @@
 
 static const int FS_CHARS_MAX = 80;  // max bytes per wrapped line
 
+// Shared, non-reentrant wrap scratch. The UI renders on a single thread, and the
+// fullscreen message view and the history list are never laid out in the same
+// frame (opening the fullscreen view early-returns before the list loop runs),
+// so one static buffer serves both. This keeps ~1.5 KB of line buffers off the
+// render-call stack (see CODE_REVIEW.md "Render-path stack peak") at the cost of
+// a fixed RAM allocation. Only used inside render()/wrap helpers — never across
+// a yield, so the single instance is safe.
+static char s_wrap_trans[512];
+static char s_wrap_lines[12][FS_CHARS_MAX];
+
 struct FullscreenMsgView {
   int  scroll;
   bool active;
+  int  _max_scroll;   // last value computed in render(); bounds KEY_DOWN
 
-  FullscreenMsgView() : scroll(0), active(false) {}
+  FullscreenMsgView() : scroll(0), active(false), _max_scroll(0) {}
 
   enum Result { NONE, PREV, NEXT, CLOSE, REPLY };
 
@@ -100,22 +111,21 @@ struct FullscreenMsgView {
     }
     display.setColor(DisplayDriver::LIGHT);
 
-    char trans_text[512];
-    display.translateUTF8ToBlocks(trans_text, body, sizeof(trans_text));
-    char lines[12][FS_CHARS_MAX];
-    int lcount = wrapLines(display, trans_text, max_px, lines, 12);
+    display.translateUTF8ToBlocks(s_wrap_trans, body, sizeof(s_wrap_trans));
+    int lcount = wrapLines(display, s_wrap_trans, max_px, s_wrap_lines, 12);
     // Reserve the right-edge column for the scroll indicator and re-wrap so text
     // can't run under it. Reserve appears only once the list overflows, and a
     // narrower second pass only ever yields more lines — so it stays overflowing.
     int reserve = scrollIndicatorReserve(display, lcount, visible);
     if (reserve > 0)
-      lcount = wrapLines(display, trans_text, max_px - reserve, lines, 12);
+      lcount = wrapLines(display, s_wrap_trans, max_px - reserve, s_wrap_lines, 12);
     int max_scroll = lcount > visible ? lcount - visible : 0;
+    _max_scroll = max_scroll;
     if (scroll > max_scroll) scroll = max_scroll;
 
     for (int i = 0; i < visible && (scroll + i) < lcount; i++) {
       display.setCursor(0, startY + i * lineH);
-      display.print(lines[scroll + i]);
+      display.print(s_wrap_lines[scroll + i]);
     }
     drawScrollIndicator(display, startY, visible * lineH, lcount, visible, scroll);
     const int nav_y = display.height() - lineH;
@@ -132,7 +142,7 @@ struct FullscreenMsgView {
 
   Result handleInput(char c) {
     if (c == KEY_UP)          { if (scroll > 0) scroll--; return NONE; }
-    if (c == KEY_DOWN)        { scroll++; return NONE; }
+    if (c == KEY_DOWN)        { if (scroll < _max_scroll) scroll++; return NONE; }
     if (c == KEY_LEFT)        return NEXT;
     if (c == KEY_RIGHT)       return PREV;
     if (c == KEY_CONTEXT_MENU) return REPLY;