feat: side-by-side comparison view

[nikazzio]

Summary

Allow users to view two pages (or two manuscripts) side by side in the Studio viewer for comparison purposes.

Motivation

Common scholarly use cases:

• Compare recto/verso of the same folio
• Compare the same text across two different manuscript witnesses
• Compare original page with its transcription rendering
• Compare pre/post restoration images

Proposed approach

• Split the Studio viewer into two synchronized panes.
• Each pane independently selects manifest + page.
• Optional synchronized scrolling/zoom (toggle on/off).
• Reuse Mirador's multi-window capability if using Mirador, or implement a dual OpenSeadragon layout.

Acceptance criteria

• "Compare" button opens split view
• Each pane can independently navigate pages
• Optional synchronized pan/zoom
• Can compare pages from different manuscripts
• Exit comparison returns to single-pane view
• Responsive layout — stacks vertically on narrow screens

Technical notes

• OpenSeadragon supports multiple viewers on the same page.
• Mirador natively supports multi-window — may be simpler if Mirador is already integrated.
• Consider a lightweight approach first: two <div> side by side, each with its own viewer instance.