Move the 128×13 zephcore_logo bitmap from ui-button/ui_pages.c into
the shared ui_common.c (linkage extern), with the dimensions and
declaration in display.h. Both UI splash renders now share the same
data — no duplicated array.
Joystick SplashScreen::render() now draws the wordmark at the top,
"MeshCore on Zephyr" beneath it, and the build date below — matching
the button-UI layout. Replaces the earlier text-only "MeshCore /
<version> / <date>" placeholder.