Add a dedicated viewer for images, tables, and diagrams

[mokhov]

Problem

Embedded visual content can be too small or complex to inspect within the normal paginated text column.

Proposed behavior

Open images, tables, and diagrams in a focused viewer with appropriate zoom and navigation controls.

Acceptance criteria

• Activating supported embedded content opens a modal viewer without changing reading position.
• The viewer supports touch and keyboard zoom, pan, reset, and close actions.
• Large tables remain readable without shrinking text below an accessible size.
• Screen-reader labels and original alternative text are preserved.
• Closing restores focus and the exact reading anchor that opened the viewer.