feat: runtime font switcher — Default vs Lemon in Settings > Display

- Add Lemon font (LemonFont.h, LemonIcons.h) to the standard build
  alongside the Adafruit built-in font; ~101 KB added to flash
- SH1106Display: add _use_lemon flag + setLemonFont(bool) to switch
  at runtime; print() and getTextWidth() use Lemon path when active;
  translateUTF8ToBlocks() passes UTF-8 through unchanged (Lemon supports
  full Unicode U+0020-U+04FF) vs transliterating for the default font
- DisplayDriver: add getCharWidth()/getLineHeight() virtuals (defaults 6/8);
  SH1106Display returns 5/9 for Lemon, 6/8 for default; add setLemonFont()
  no-op virtual; fix orphaned UTF-8 leading byte in drawTextEllipsized
- FullscreenMsgView: replace fixed-char-count word-wrap with pixel-accurate
  wrap using display.getTextWidth(); use getLineHeight() for spacing so
  Lemon (9 px) and default (8 px) both render correctly
- NodePrefs: add use_lemon_font field (0=default, 1=Lemon)
- SettingsScreen: add "Font" item in Display section (Default / Lemon),
  calls UITask::applyFont() on change
- UITask: add applyFont() that calls display->setLemonFont(use_lemon_font);
  called at startup (begin()) and when the setting changes

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

examples/companion_radio/NodePrefs.h

@@ -78,4 +78,5 @@ struct NodePrefs {  // persisted to file
   struct DmMelodyEntry { uint8_t prefix[4]; uint8_t slot; }; // slot: 0=global,1=melody1,2=melody2
   static const int DM_MELODY_TABLE_MAX = 16;
   DmMelodyEntry dm_melody[DM_MELODY_TABLE_MAX];
+  uint8_t  use_lemon_font;  // 0=default Adafruit font, 1=Lemon font (Unicode, pixel-accurate wrap)
 };