Document UI/UX rendering customization

[Zhortein]

Objective

Document the UI/UX customization features introduced in this milestone.

Context

After action display modes, boolean display modes, sortable header polish, layout options and custom CSS classes are implemented, documentation must be updated.

Scope

• Update or create UI/UX rendering customization documentation.
• Document action icons.
• Document action display modes.
• Document boolean cell display modes.
• Document sortable header indicators.
• Document layout options.
• Document custom CSS class options.
• Update README/docs index if needed.
• Update roadmap for 0.16 completion.

Out of scope

• Column header filter popover implementation docs if only designed.
• Full theme registry documentation.
• Tailwind support.

Constraints

• Documentation must match implemented behavior.
• Examples must be generic.
• QA passes.

Acceptance criteria

• UI/UX customization docs are updated.
• README/docs index links are accurate.
• Roadmap is updated for 0.16 completion.
• QA passes.