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/bin/moonarch-vpn

@@ -1,5 +1,5 @@
 #!/usr/bin/env bash
-# ABOUTME: Rofi-based VPN connection manager using NetworkManager (nmcli).
+# ABOUTME: Walker-dmenu VPN connection manager using NetworkManager (nmcli).
 # ABOUTME: Lists VPN connections and toggles them on/off.
 
 # Copyright (C) 2021  Damien Cassou
@@ -80,7 +80,7 @@ function extract_connection_name_from_result() {
 # used as lines to select from.
 function start_rofi() {
     local content="$1"
-    echo -e "$content" | rofi -dmenu -theme /etc/xdg/rofi/themes/vpn.rasi -mesg "󰖂  VPN Connection Manager" -icon "󰖂"
+    echo -e "$content" | walker -d -p "󰖂  VPN"
 }
 
 # List the VPN connections and let the user toggle one using rofi.