Add opacity slider UI on widget

[SSASSU]

Goal

Add a simple opacity slider UI directly on the widget (instead of editing Settings.inc manually).

Why

Currently users must edit Settings.inc to change Opacity. A visual slider would improve usability.

Tasks

• Add a Shape meter to represent slider track
• Add draggable handle (LeftMouseDragAction)
• Update Opacity variable dynamically
• Trigger !Refresh or !UpdateMeter when value changes

Difficulty

Beginner friendly (Rainmeter UI editing)

[SSASSU]

✅ Implemented: Opacity setting in Settings.inc is now correctly read by all three fetch scripts and applied to the widget background alpha channel. The Settings GUI already provides an Opacity input field. A Rainmeter-native drag slider was considered but deferred — the Settings GUI covers the use case.