wallpaper: row-per-notch wheel and a sticky folder header

The thumbnail list scrolled a fraction of a thumbnail per wheel notch,
which is ListView's default step and slow with rows this tall. A
MouseArea over the list (acceptedButtons: Qt.NoButton, so clicks and
hovers still reach the thumbnails) now moves one row per notch, and
keeps a touchpad's pixel delta as it is.

An open folder with many images scrolled its header out of view, so
folding it meant scrolling back to the top. A copy of the header is now
pinned over the list while the topmost row belongs to a folder whose own
header has scrolled off; clicking it folds that folder and positions the
view at its header. The header row became an inline component so the
list and the pinned copy draw the same thing.

Loads clean; not yet confirmed by eye.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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