feat(rlcd): runtime font size selector in Settings->Display

- device.properties bump default to 20
- lvgl_fonts add runtime override API (g_runtime_font_size + externs for all Montserrat sizes)
- DisplaySettings add fontSize field persisted in properties
- Display app adds Font size dropdown 14-28 applying immediately
- Lvgl.cpp applies saved runtime font on boot
- device.py new bucket <=20 includes all fonts for runtime switching (~150KB)

Co-authored-by: internal-model

Tactility/Include/Tactility/settings/DisplaySettings.h

@@ -26,6 +26,7 @@ struct DisplaySettings {
     bool disableScreensaverWhenCharging = false;
     bool invertColor = false;
     uint8_t monochromeThreshold = 60;
+    uint8_t fontSize = 0; // 0 = use compile-time default, otherwise 14/16/18/20/24/28
 };
 lv_display_rotation_t toLvglDisplayRotation(Orientation orientation);
 bool load(DisplaySettings& settings);