Add window pinning support

[Nekit678]

Add support for pinning individual windows to the physical screen so they stay fixed while the canvas pans or zooms.

This PR:

• adds screen-pinned window state to the canvas model and window manager
• skips pinned windows during canvas reprojection, reset, clipping, and foreground recentering
• lets users toggle pinning from the search overlay with inline per-window checkboxes and Space
• keeps pinned, maximized, and supported screen-fixed windows visible during pan overview
• adds explicit MyDockFinder handling so dock/finder panels are raised above the panning overlay
• adds a tray/config toggle for showing screen-fixed windows while panning

[oreyg]

I would need some time to read this though, but brief read lgtm