Add modular interactive widgets to the Quickshell bar

Introduces a widgets/ directory of first-party QML modules and a common/
helpers library. Widgets auto-load by name from
default/quickshell/bar/widgets/<name>.qml — no edits to shell.qml needed
for additions beyond the firstPartyWidgets registry.

New widgets (all orientation-aware, top/bottom/left/right):
- media: MPRIS now-playing with scrolling label, cover art popup,
  play/pause/skip controls
- audioPanel: master volume slider, output device picker, per-app mixer
- networkPanel: Wi-Fi scan + connect, current connection details
- bluetoothPanel: paired device list, connect/disconnect, battery
- calendar: month grid popup keyed to today
- notificationCenter: live notification feed, DND toggle, clear all
- brightness: slider popup with scroll-to-adjust
- powerProfile: power-saver/balanced/performance picker (UPower)
- systemStats: inline CPU + RAM sparklines, expanded popup with btop
- weatherFlyout: weather popup with refresh and full report
- workspacesPro: animated focus indicator that slides between workspaces
- powerMenu: lock/suspend/log out/reboot/shutdown popup
- idleInhibitor: coffee-cup toggle wired to omarchy-toggle-idle
- microphone: mic state + mute toggle + scroll volume

Common helpers (default/quickshell/bar/common/):
- WidgetButton: hover/press scale animation, tooltip integration
- PopupCard: orientation-aware anchor + slide-in opacity transition
- Slider: drag/wheel-able linear slider with knob hover bump
- PillButton: rounded action button with icon + label

Bar root gains requestPopout/releasePopout so only one popup is open at
a time across widgets.

Updates bar-defaults.json to ship the new widgets out of the box while
keeping the legacy modules available.

default/quickshell/bar/bar-defaults.json

@@ -1,13 +1,18 @@
 {
   "position": "top",
   "fontFamily": "JetBrainsMono Nerd Font",
-  "centerAnchor": "clock",
+  "centerAnchor": "calendar",
   "layout": {
-    "left": ["omarchy", "workspaces"],
-    "center": ["clock", "weather", "update", "voxtype", "screenRecording", "idle", "notifications"],
-    "right": ["tray", "bluetooth", "network", "audio", "cpu", "battery"]
+    "left": ["omarchy", "workspacesPro"],
+    "center": ["media", "calendar", "weatherFlyout", "update", "voxtype", "screenRecording", "idle"],
+    "right": ["tray", "systemStats", "notificationCenter", "microphone", "bluetoothPanel", "networkPanel", "audioPanel", "brightness", "powerProfile", "battery", "powerMenu"]
   },
   "modules": {
+    "calendar": {
+      "format": "dddd HH:mm",
+      "formatAlt": "dd MMMM 'W'ww yyyy",
+      "verticalFormat": "HH\n—\nmm"
+    },
     "clock": {
       "format": "dddd HH:mm",
       "formatAlt": "dd MMMM 'W'ww yyyy",