v10.5: Responsive Global Search + per-column hide + dark group bars

Highlights

Global Search no longer freezes the UI.
The Search dialog now runs the MIX walk on a background thread, so you
can drag, resize, or close the window while results are being gathered.
The Search button doubles as Cancel mid-search. A progress line
under the buttons shows "Found N - searching..." live, then
"N results." on completion (auto-clears after a moment). On large
result sets, columns now size to the header instead of measuring every
row's content, killing the second freeze that used to happen right
after the walk finished.

Right-click any column header to hide/show columns.
Explorer-style checklist popup on both the main panes and either Search
dialog. Click a name to toggle that column's visibility - state
persists across sessions, per listview. Pair it with the new
Theme -> Show Column Headers toggle that hides the whole header
band if you want a cleaner view.

Group headers in the Global Search dialog now read clearly in dark
mode. White text on the dark theme strip, with the OS-native
expand/collapse chevron drawn via the TreeView theme glyph so it
matchex Explorer's look in both light and dark.