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.

legacy/rofi/themes/settings-menu.rasi

@@ -0,0 +1,26 @@
+@import                          "../config.rasi"
+@import                          "../colors.rasi"
+
+
+listview {
+    columns:                     4;
+    lines:                       4;
+}
+
+inputbar {
+    children:                    [ "textbox-prompt-colon", "entry"];
+}
+
+element {
+    orientation:vertical;
+    children: ["element-icon", "element-text"];
+}
+element-icon {
+    size: 32px;
+}
+
+element-text {
+    vertical-align:              0.5;
+    horizontal-align:            0.5;
+    font: "UbuntuSans Nerd Font 9";
+}