Add controlCenter widget: one-click quick settings popup

Single bar button (md-tune-vertical glyph) that opens a popup with the
controls most users reach for repeatedly throughout the day:

- Volume slider + mute toggle (Pipewire defaultAudioSink)
- Brightness slider (brightnessctl, coalesced via writeTimer)
- 2x2 tile grid: Do Not Disturb (makoctl mode), Night Light (omarchy-
  toggle-nightlight, temperature probed), Keep Awake (hypridle pid
  detection), Theme (current symlink + opens omarchy menu themes)
- Power-profile row (Saver / Balanced / Performance), hidden when
  power-profiles-daemon is not installed

Polled every 3s via Process + StdioCollector pairs gated by !running
busy checks. Default layout adds it before powerMenu on the right.

default/quickshell/bar/shell.qml

@@ -279,7 +279,8 @@ ShellRoot {
     "nightLight": true,
     "keyboardLayout": true,
     "lockKeys": true,
-    "spacer": true
+    "spacer": true,
+    "controlCenter": true
   })
 
   function firstPartyWidgetSource(name) {