Implement `<textarea>` caret rendering and behavior

[FlorianRappl]

Description
<textarea> lacks caret rendering/positioning behavior required for realistic text editing representation.

Scope

• Render caret in textarea content at the correct logical/visual position.
• Handle line wrapping/newlines for caret placement.
• Integrate with selection/edit state model as currently supported.

Acceptance criteria

• Caret is visible and correctly positioned for single-line and multi-line content.
• Position updates with text/line changes.
• Tests cover wrapped lines, empty content, and trailing newline cases.