Add bi-directional mouse wheel scrolling to image picker

[jvlianodorneles]

Summary

Enables mouse wheel and trackpad scroll navigation in ImagePicker.qml (used by omarchy theme bg-switcher and omarchy theme switcher).

Details

• Adds onWheel handlers to the image thumbnail items, the central card, and the backdrop scrim so scrolling works anywhere over the picker.
• Implements bi-directional scrolling:
  • Scroll down / scroll right -> advances to the next item (selectAdjacent(1))
  • Scroll up / scroll left -> returns to the previous item (selectAdjacent(-1))
• Handles discrete mouse wheel notches (such as standard 120-unit ticks) with direct 1:1 stepping, and accumulates fine-grained touchpad deltas (pixelDelta / fractional angleDelta) with an inactivity reset timer.