refactor(ui): dedupe UTF-8 codepoint decoder

Three copies of the same routine: DisplayDriver::decodeCodepoint (added
in the FullscreenMsgView wrap fix) plus per-driver duplicates in
SH1106Display and GxEPDDisplay. Consolidate on DisplayDriver and have
translateUTF8Static also route through it instead of its own inline
decoder. Net: 63 lines removed, single source of truth.

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

src/helpers/ui/DisplayDriver.h

@@ -134,23 +134,12 @@ public:
     size_t j = 0;
     const uint8_t* p = (const uint8_t*)src;
     while (*p && j < dest_size - 1) {
-      uint8_t c = *p++;
-      if (c < 0x80) {
+      if (*p < 0x80) {
+        uint8_t c = *p++;
         if (c >= 32) dest[j++] = c;
       } else {
-        uint32_t cp = c;
-        if ((c & 0xE0) == 0xC0) {
-          cp = c & 0x1F;
-          if (*p) cp = (cp << 6) | (*p++ & 0x3F);
-        } else if ((c & 0xF0) == 0xE0) {
-          cp = c & 0x0F;
-          if (*p) cp = (cp << 6) | (*p++ & 0x3F);
-          if (*p) cp = (cp << 6) | (*p++ & 0x3F);
-        } else {
-          while (*p && (*p & 0xC0) == 0x80) p++;
-          dest[j++] = '\xDB';
-          continue;
-        }
+        uint32_t cp = decodeCodepoint(p);
+        // transliterateCodepoint already returns '\xDB' for unmapped values (incl. 0xFFFD).
         dest[j++] = transliterateCodepoint(cp);
       }
     }

src/helpers/ui/GxEPDDisplay.cpp

@@ -23,31 +23,6 @@ static int fontAscender(int sz, bool use_lemon, int scale) {
   return 0;                                     // GFX built-in font: cursor is top-left of cell
 }
 
-uint32_t GxEPDDisplay::decodeUtf8(const uint8_t*& p) {
-  uint8_t c = *p++;
-  if (c < 0x80) return c;
-  if ((c & 0xE0) == 0xC0) {
-    uint32_t cp = c & 0x1F;
-    if (*p) cp = (cp << 6) | (*p++ & 0x3F);
-    return cp;
-  }
-  if ((c & 0xF0) == 0xE0) {
-    uint32_t cp = c & 0x0F;
-    if (*p) cp = (cp << 6) | (*p++ & 0x3F);
-    if (*p) cp = (cp << 6) | (*p++ & 0x3F);
-    return cp;
-  }
-  if ((c & 0xF8) == 0xF0) {
-    uint32_t cp = c & 0x07;
-    if (*p) cp = (cp << 6) | (*p++ & 0x3F);
-    if (*p) cp = (cp << 6) | (*p++ & 0x3F);
-    if (*p) cp = (cp << 6) | (*p++ & 0x3F);
-    return cp;
-  }
-  while (*p && (*p & 0xC0) == 0x80) p++;
-  return 0xFFFD;
-}
-
 // y is the GFX baseline (display.getCursorY()), which equals original_y + 8*sc.
 // Pixels are placed at y + yo*sc + row*sc — identical to how GFX would render
 // a scaled GFX font, but bypassing GFX so multi-byte UTF-8 is decoded correctly.
@@ -209,7 +184,7 @@ void GxEPDDisplay::print(const char* str) {
     const int sc = scale();
     const uint8_t* p = (const uint8_t*)str;
     while (*p) {
-      uint32_t cp = decodeUtf8(p);
+      uint32_t cp = decodeCodepoint(p);
       if (cp == '\n') { cy += Lemon.yAdvance * sc; cx = 0; }
       else            { cx = drawLemonChar(cx, cy, cp, sc); }
     }
@@ -271,7 +246,7 @@ uint16_t GxEPDDisplay::getTextWidth(const char* str) {
     uint16_t total = 0;
     const int sc = scale();
     const uint8_t* p = (const uint8_t*)str;
-    while (*p) total += lemonXAdvance(decodeUtf8(p), sc);
+    while (*p) total += lemonXAdvance(decodeCodepoint(p), sc);
     return total;
   }
   display.setTextWrap(false);

src/helpers/ui/GxEPDDisplay.h

@@ -57,7 +57,6 @@ class GxEPDDisplay : public DisplayDriver {
   uint8_t _full_refresh_interval = 0;
   uint8_t _partial_count = 0;
 
-  static uint32_t decodeUtf8(const uint8_t*& p);
   int16_t drawLemonChar(int16_t x, int16_t y, uint32_t cp, int sc);
   uint8_t lemonXAdvance(uint32_t cp, int sc);
   int scale() const { return (width() >= height()) ? 2 : 1; }

src/helpers/ui/SH1106Display.cpp

@@ -65,31 +65,6 @@ void SH1106Display::setCursor(int x, int y)
   display.setCursor(x, y);
 }
 
-uint32_t SH1106Display::decodeUtf8(const uint8_t*& p) {
-  uint8_t c = *p++;
-  if (c < 0x80) return c;
-  if ((c & 0xE0) == 0xC0) {
-    uint32_t cp = c & 0x1F;
-    if (*p) cp = (cp << 6) | (*p++ & 0x3F);
-    return cp;
-  }
-  if ((c & 0xF0) == 0xE0) {
-    uint32_t cp = c & 0x0F;
-    if (*p) cp = (cp << 6) | (*p++ & 0x3F);
-    if (*p) cp = (cp << 6) | (*p++ & 0x3F);
-    return cp;
-  }
-  if ((c & 0xF8) == 0xF0) {
-    uint32_t cp = c & 0x07;
-    if (*p) cp = (cp << 6) | (*p++ & 0x3F);
-    if (*p) cp = (cp << 6) | (*p++ & 0x3F);
-    if (*p) cp = (cp << 6) | (*p++ & 0x3F);
-    return cp;
-  }
-  while (*p && (*p & 0xC0) == 0x80) p++;
-  return 0xFFFD;
-}
-
 int16_t SH1106Display::drawLemonChar(int16_t x, int16_t y, uint32_t cp) {
   int sz = _text_sz;
   for (uint8_t i = 0; i < lemonIconCount; i++) {
@@ -161,7 +136,7 @@ void SH1106Display::print(const char *str)
   int16_t cy = display.getCursorY();
   const uint8_t* p = (const uint8_t*)str;
   while (*p) {
-    uint32_t cp = decodeUtf8(p);
+    uint32_t cp = decodeCodepoint(p);
     if (cp == '\n') { cy += Lemon.yAdvance; cx = 0; }
     else            { cx = drawLemonChar(cx, cy, cp); }
   }
@@ -188,7 +163,7 @@ uint16_t SH1106Display::getTextWidth(const char *str)
   if (_use_lemon) {
     uint16_t width = 0;
     const uint8_t* p = (const uint8_t*)str;
-    while (*p) width += lemonXAdvance(decodeUtf8(p));
+    while (*p) width += lemonXAdvance(decodeCodepoint(p));
     return width;
   }
   int16_t x1, y1;

src/helpers/ui/SH1106Display.h

@@ -25,7 +25,6 @@ class SH1106Display : public DisplayDriver
   int  _text_sz;
 
   bool i2c_probe(TwoWire &wire, uint8_t addr);
-  static uint32_t decodeUtf8(const uint8_t*& p);
   int16_t drawLemonChar(int16_t x, int16_t y, uint32_t cp);
   uint8_t lemonXAdvance(uint32_t cp);