Replace Rofi with Walker as application launcher

Walker (GTK4 + Elephant backend) replaces rofi-lbonn-wayland-git as the
central launcher and menu framework.

Native Walker providers replace 5 custom rofi scripts:
- App launcher (desktopapplications provider)
- Clipboard (clipboard provider, replaces cliphist frontend)
- Bluetooth (bluetooth provider, replaces bluetoothctl script)
- Volume/audio (wireplumber provider)
- Sink switcher (wireplumber provider)

3 scripts ported to Walker dmenu mode:
- moonarch-vpn (nmcli)
- moonarch-cpugov (auto-cpufreq)
- moonarch-sink-switcher (pactl)

Settings menu (moonarch-setmen) removed — apps are findable via Walker
app search directly.

Walker theme (gtk-inherit) inherits all colors from the active GTK4
theme instead of hardcoding Catppuccin values.

Walker and Elephant run as systemd user services for instant startup.

Also standardizes GTK theme to Colloid-Grey-Dark-Catppuccin across all
config files (was inconsistent between gsettings and file configs).

Old rofi configs preserved in legacy/rofi/ for reference.

defaults/xdg/niri/config.kdl

@@ -122,21 +122,20 @@ window-rule {
 binds {
     Mod+Shift+Slash { show-hotkey-overlay; }
 
-    Super+C hotkey-overlay-title=null { spawn "moonarch-clipboard"; }
+    Super+C hotkey-overlay-title=null { spawn "walker" "-s" "clipboard"; }
 
     Alt+W { spawn-sh "killall waybar && waybar &"; }
 
     Super+E { spawn-sh "xdg-open ~"; }
 
-    Super+T { spawn-sh "pkill rofi || rofi -show fb -modes \"fb:moonarch-setmen\" -theme /etc/xdg/rofi/themes/settings-menu.rasi"; }
 
-    Super+N { spawn-sh "pkill rofi || moonarch-vpn"; }
+    Super+N { spawn-sh "moonarch-vpn"; }
 
     Mod+Return hotkey-overlay-title="Open a Terminal: foot" { spawn "foot"; }
-    Mod+Space hotkey-overlay-title="Run an Application: rofi" { spawn-sh "pkill rofi || moonarch-launcher"; }
+    Mod+Space hotkey-overlay-title="Run an Application: walker" { spawn "walker"; }
     Super+Alt+L hotkey-overlay-title="Session Menu: moonset" { spawn "moonset"; }
 
-    Mod+A { spawn-sh "pkill rofi || moonarch-volume"; }
+    Mod+A { spawn "walker" "-s" "audio"; }
 
     XF86AudioRaiseVolume allow-when-locked=true { spawn-sh "wpctl set-volume @DEFAULT_AUDIO_SINK@ 0.1+"; }
     XF86AudioLowerVolume allow-when-locked=true { spawn-sh "wpctl set-volume @DEFAULT_AUDIO_SINK@ 0.1-"; }