feat: support transparency in fluid colors

[jayf0x]

Goal

Allow transparent/semi-transparent fluid colors so the canvas background (or page behind it) shows through.

Context

Details TBD — this is a placeholder to track the request. Before implementing, clarify:

• Which color props should support alpha? (fluid color, background, or both?)
• Should the canvas itself be transparent (alpha: true in the WebGL context)?
• How does transparency interact with the specular/shine pass?

Acceptance criteria

• At least one color prop accepts an rgba value or alpha channel
• Canvas background is visible through transparent fluid regions
• Specular highlights still render correctly over transparent fluid
• Tests updated

Files likely touched: src/globals.d.ts, src/core/config.ts, src/core/shaders.ts, src/core/simulation.ts