Fix popup graphic and text overlap on taller screens

[owenb321]

Summary

• The "Time's Up!" and "Timer Deleted" popups centered the PDC/bitmap graphic
  vertically while anchoring the text layer at a hardcoded y=125. On taller
  platforms (Emery, Gabbro) the centered graphic shifted past y=125 and
  overlapped the title text.
• layers_center_in_window now keeps the graphic in its original centered
  position so authored animations (alarm rings, shredder shreds) still fit on
  screen, and places the text directly below the graphic with a 5px gap,
  clamped to the screen bottom.
• The original Aplite -7 vertical offset is preserved.

Fixes issue #4