Implement dial-style overlay UI for openwheel gadget

[fredaime]

Motivation

• Replace the placeholder gadget UI with a transient dial overlay that mirrors global dial behavior (mode switching, scroll-to-adjust, click-to-change-mode, and auto-hide).
• Provide an embeddable PlasmoidItem that can be integrated with the existing gadget package to present a visual dial and value feedback.

Description

• Replaced openwheel-gadget/package/contents/ui/main.qml placeholder with a PlasmoidItem-based dial overlay implementing properties modeIndex, currentValue, and overlayVisible.
• Added a ListModel of modes, WheelHandler for scroll adjustments, TapHandler for mode cycling, and a Timer (hideTimer) to auto-hide the overlay.
• Implemented custom drawing in a Canvas to render the outer ring, accent arc, and tick marks, and added Component.onCompleted: showOverlay() to show the overlay on startup.
• Updated UI elements to reflect mode icon/name/value and wired Connections to call dialCanvas.requestPaint() on relevant changes.

Testing

• No automated tests were executed; this is a UI-only change and was committed after functional QML changes.

---

Codex Task