[FEATURE] Add Solid Layer Element and etc.

Problem

Currently, users cannot add a customizable solid color layer to their projects. This limits creative possibilities such as creating backgrounds, color overlays without importing external images.

Solution

Add a new element type: Solid Layer.

• Users can insert a solid color rectangle (or circle, etc.) as a new layer on the timeline.
• The color should be fully customizable using a color picker or hex input.
• The layer should be resizable and movable in the canvas.
• Allow opacity/alpha adjustments.
• The solid color layer should stack with other layer types (video, image, text, etc.).

Alternative

• Import colored images as a workaround (not efficient).
• Use existing shapes like stickers, but they not except thing and may not offer full control or color selection.

Anything else?

This feature is common in video editors like Premiere Pro and CapCut and would improve OpenCut’s usability and versatility.

The issue names no files, tests, or entry points. Start by tracing how existing video, image, text, and sticker layers are represented, inserted, resized, moved, and rendered; the work is done when a customizable solid shape supports color, opacity, canvas editing, timeline stacking, and the listed layer interactions.