feat: add visual HTML block editing

[makhnatkin]

Improve HTML block editing

• Added three editing variants with element properties, inline HTML source, and a source overlay.
• Preserved the clicked text position when opening element source on double-click.
• Updated element outlines to match block selection.
• Added coverage for text, images, cancellation, and editing after scrolling.

Summary by Sourcery

Enable visual and source-based editing of HTML block elements while preserving selection context, scrolling, and responsive layout.

New Features:

• Add visual HTML block editing with element text and attribute properties, inline source editing, and source overlay editing.
• Support opening element source at the clicked text position and optionally opening block source on double-click.
• Provide configurable HTML block editing modes and expose all variants in the demo stories.

Bug Fixes:

• Preserve element position and viewport scrolling while opening editors, including after scrolling and in narrow viewports.
• Keep HTML block outlines and hover controls aligned with the selected element without shifting preview content.

Enhancements:

• Improve the HTML block toolbar and source editor presentation with responsive controls, line numbers, save/cancel actions, and localized labels.

Tests:

• Expand visual coverage for text and image editing, attributes, cancellation, source positioning, scrolling, hover behavior, responsive layouts, and pointer-move handling.
• Add unit coverage for mapping preview carets to source HTML and safely replacing matched elements.

[gravity-ui]

[gravity-ui]