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)