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.

scripts/post-install.sh

@@ -44,7 +44,7 @@ paru -S --needed --noconfirm moonarch-git
 
 # --- User-level GTK4 symlinks for libadwaita apps ---
 
-THEME_NAME="Colloid-Catppuccin"
+THEME_NAME="Colloid-Grey-Dark-Catppuccin"
 THEME_GTK4="/usr/share/themes/$THEME_NAME/gtk-4.0"
 
 if [[ -d "$THEME_GTK4" ]]; then