feat: dynamic font-adaptive layout and Lemon font icon support for SH1106

- Add getCharWidth()/getLineHeight() virtual methods to DisplayDriver so
  all layout math adapts to whatever font is active
- Fix Lemon font baseline offset from +8 to +6 (cap height, not FONT_ASCENT)
- Add LemonIcons.h with Private Use Area glyph table; U+E03B mute icon
  rendered via drawLemonChar icon lookup
- FullscreenMsgView: compute wrap width and visible lines dynamically from
  display font metrics instead of hardcoded constants
- SettingsScreen and MessagesScreen: replace hardcoded ITEM_H/VISIBLE with
  members computed at render() time from getLineHeight()

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

src/helpers/ui/DisplayDriver.h

@@ -27,6 +27,8 @@ 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; }   // advance width of a typical character (xAdvance for monospace)
+  virtual int getLineHeight() const { return 8; }  // pixel rows per line (cell height, not just glyph height)
   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);

src/helpers/ui/LemonIcons.h

@@ -0,0 +1,19 @@
+// Lemon font icon glyphs (Private Use Area), converted manually from lemon.bdf
+// Add new icons by appending to the bitmap array, lemonIconCPs, and lemonIconGlyphs.
+#pragma once
+#include <Adafruit_GFX.h>
+
+static const uint8_t lemonIconBitmaps[] PROGMEM = {
+  0xEE, 0xBA, 0x2B, 0xA3, 0xB0,  // U+E03B mute: 6×6, yoff=0
+};
+
+static const uint32_t lemonIconCPs[] PROGMEM = {
+  0xE03B,  // mute
+};
+
+static const GFXglyph lemonIconGlyphs[] PROGMEM = {
+  //  off   w   h  adv  xo   yo
+  {  0,   6,  6,   5,   0,  -6 },  // U+E03B mute
+};
+
+static const uint8_t lemonIconCount = sizeof(lemonIconCPs) / sizeof(lemonIconCPs[0]);

src/helpers/ui/SH1106Display.cpp

@@ -2,6 +2,7 @@
 #include <Adafruit_GrayOLED.h>
 #include "Adafruit_SH110X.h"
 #include "LemonFont.h"
+#include "LemonIcons.h"
 
 bool SH1106Display::i2c_probe(TwoWire &wire, uint8_t addr)
 {
@@ -90,6 +91,27 @@ uint32_t SH1106Display::decodeUtf8(const uint8_t*& p) {
 
 // Draw one Lemon glyph at (x, y), return new cursor x.
 int16_t SH1106Display::drawLemonChar(int16_t x, int16_t y, uint32_t cp) {
+  // Check icon table for Private Use Area codepoints
+  for (uint8_t i = 0; i < lemonIconCount; i++) {
+    if (pgm_read_dword(&lemonIconCPs[i]) == cp) {
+      const GFXglyph* g = &lemonIconGlyphs[i];
+      uint8_t  w  = pgm_read_byte(&g->width);
+      uint8_t  h  = pgm_read_byte(&g->height);
+      int8_t   xo = (int8_t)pgm_read_byte(&g->xOffset);
+      int8_t   yo = (int8_t)pgm_read_byte(&g->yOffset);
+      uint8_t  xa = pgm_read_byte(&g->xAdvance);
+      uint16_t bo = pgm_read_word(&g->bitmapOffset);
+      uint8_t bits = 0, bit = 0;
+      for (uint8_t row = 0; row < h; row++)
+        for (uint8_t col = 0; col < w; col++) {
+          if (!bit) { bits = pgm_read_byte(&lemonIconBitmaps[bo++]); bit = 0x80; }
+          if (bits & bit) display.drawPixel(x + xo + col, y + 6 + yo + row, _color);
+          bit >>= 1;
+        }
+      return x + xa;
+    }
+  }
+
   if (cp < Lemon.first || cp > Lemon.last) {
     // outside font range — use default advance
     return x + 6;
@@ -107,7 +129,7 @@ int16_t SH1106Display::drawLemonChar(int16_t x, int16_t y, uint32_t cp) {
     for (uint8_t col = 0; col < w; col++) {
       if (!bit) { bits = pgm_read_byte(&lemonBitmaps[bo++]); bit = 0x80; }
       if (bits & bit)
-        display.drawPixel(x + xo + col, y + yo + row, _color);
+        display.drawPixel(x + xo + col, y + 6 + yo + row, _color);  // +6 = cap height: cursor y aligns with top of uppercase/digit glyphs
       bit >>= 1;
     }
   }

src/helpers/ui/SH1106Display.h

@@ -45,6 +45,8 @@ public:
   void drawRect(int x, int y, int w, int h) override;
   void drawXbm(int x, int y, const uint8_t *bits, int w, int h) override;
   uint16_t getTextWidth(const char *str) override;
+  int getCharWidth() const override { return 5; }   // Lemon font: xAdvance = 5
+  int getLineHeight() const override { return 9; }  // Lemon font: visual line spacing (yAdvance=10, used as 9 for tighter fit)
   void setBrightness(uint8_t level) override;
   void endFrame() override;