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/common/PopupCard.qml

@@ -0,0 +1,93 @@
+import QtQuick
+import Quickshell
+
+PopupWindow {
+  id: root
+
+  required property Item anchorItem
+  required property QtObject bar
+  property var owner: null
+  property int margin: 8
+  property int padding: 14
+  property int contentWidth: 280
+  property int contentHeight: 200
+  property bool dismissOnOutsideClick: true
+  property bool open: false
+
+  onOpenChanged: {
+    if (!bar) return
+    if (open) bar.requestPopout(owner || root)
+    else if (bar.activePopout === (owner || root)) bar.releasePopout(owner || root)
+  }
+
+  default property alias contentItem: contentHolder.children
+
+  visible: open
+  color: "transparent"
+  implicitWidth: contentWidth
+  implicitHeight: contentHeight
+
+  anchor {
+    id: popupAnchor
+    window: anchorItem ? anchorItem.QsWindow.window : null
+    adjustment: PopupAdjustment.Slide
+    edges: Edges.Top | Edges.Left
+    gravity: Edges.Bottom | Edges.Right
+    rect.width: 1
+    rect.height: 1
+
+    onAnchoring: {
+      if (!root.anchorItem || !root.bar) return
+
+      var target = root.anchorItem
+      var popupWidth = root.implicitWidth
+      var popupHeight = root.implicitHeight
+      var localX = target.width / 2 - popupWidth / 2
+      var localY = target.height + root.margin
+
+      if (root.bar.position === "bottom") {
+        localY = -popupHeight - root.margin
+      } else if (root.bar.position === "left") {
+        localX = target.width + root.margin
+        localY = target.height / 2 - popupHeight / 2
+      } else if (root.bar.position === "right") {
+        localX = -popupWidth - root.margin
+        localY = target.height / 2 - popupHeight / 2
+      }
+
+      var window = target.QsWindow.window
+      if (!window) return
+
+      var point = window.contentItem.mapFromItem(target, localX, localY)
+      popupAnchor.rect.x = Math.round(point.x)
+      popupAnchor.rect.y = Math.round(point.y)
+    }
+  }
+
+  Rectangle {
+    id: card
+    anchors.fill: parent
+    color: root.bar ? root.bar.background : "#101315"
+    border.color: root.bar ? root.bar.foreground : "#cacccc"
+    border.width: 1
+    radius: 0
+    opacity: root.open ? 0.97 : 0
+
+    Behavior on opacity {
+      NumberAnimation { duration: 140; easing.type: Easing.OutCubic }
+    }
+
+    Item {
+      id: contentHolder
+      anchors.fill: parent
+      anchors.margins: root.padding
+    }
+
+    MouseArea {
+      anchors.fill: parent
+      acceptedButtons: Qt.NoButton
+      hoverEnabled: false
+      propagateComposedEvents: true
+    }
+  }
+}