Mod-agnostic compositor binds: avoid Alt in Mod combos

If a Wayland compositor's Mod key differs per host (Super on one, Alt on another), any Mod+Alt+X bind collapses to Alt+X on the Alt host and silently duplicates Mod+X. Instead of gating those binds with conditionals, rebind them to keys with no Alt, e.g. Mod+BracketLeft/Mod+BracketRight plus Shift/Ctrl variants for monitor focus/move. Both niri and umbriel accept xkb keysym names like BracketLeft. Validate generated configs: niri validate -c <config.kdl> and umbriel config validate -c <config.toml> (build home.activationPackage and look under home-files/.config).