niri: working minimize via declared stash workspace

Mod+M referenced an undeclared 'minimized' workspace, so it silently did
nothing. Declare it, add focus=false so focus does not follow the window
into the stash, and point Mod+Shift+M at the restore picker.

niri pins declared named workspaces to index 1 and will not move them, so
Mod+1..9 are offset by one.

install.sh

@@ -55,6 +55,7 @@ ln -sf "$(pwd)/scripts/screenshot.sh" ~/.local/bin/screenshot
 ln -sf "$(pwd)/scripts/volume.sh" ~/.local/bin/volume
 ln -sf "$(pwd)/scripts/gamemode-session.sh" ~/.local/bin/gamemode-session
 ln -sf "$(pwd)/scripts/gamemode-watch.py" ~/.local/bin/gamemode-watch
+ln -sf "$(pwd)/scripts/window-restore.py" ~/.local/bin/window-restore
 
 echo "==> Enabling systemd user services"
 mkdir -p ~/.config/systemd/user