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>

src/helpers/ui/DisplayDriver.h

@@ -27,6 +27,9 @@ public:
   virtual void drawRect(int x, int y, int w, int h) = 0;
   virtual void drawXbm(int x, int y, const uint8_t* bits, int w, int h) = 0;
   virtual uint16_t getTextWidth(const char* str) = 0;
+  virtual int getCharWidth() const { return 6; }   // typical character advance width (px)
+  virtual int getLineHeight() const { return 8; }  // pixel rows per text line
+  virtual void setLemonFont(bool) { }              // no-op; overridden by displays that support Lemon
   virtual void drawTextCentered(int mid_x, int y, const char* str) {   // helper method (override to optimise)
     int w = getTextWidth(str);
     setCursor(mid_x - w/2, y);
@@ -182,6 +185,10 @@ public:
     while (str_len > 0 && getTextWidth(temp_str) > max_width - ellipsis_width) {
       temp_str[--str_len] = 0;
     }
+    // Strip orphaned UTF-8 leading byte left by byte-at-a-time trimming above.
+    while (str_len > 0 && ((uint8_t)temp_str[str_len - 1] & 0xC0) == 0xC0) {
+      temp_str[--str_len] = 0;
+    }
     strcat(temp_str, ellipsis);
     
     setCursor(x, y);