0.4.0: Content-editing UI (annotate/highlight/draw)

[yfedoseev]

Goal

Edit and add text and images — UI layer on top of the existing core engine, across all viewer platforms.

Feature scope

• Edit existing text runs
• Add text / images
• Manage resources (fonts, images)
  Core: src/editor/ (document_editor, dom, resource_manager), writer/

Platform checklist (build the UI on each viewer)

• .NET desktop — WPF / WinUI 3 / Avalonia / MAUI
• Web — core viewer + React / Vue / Angular / Svelte
• Mobile/native — SwiftUI / Jetpack Compose / Flutter
• Shells — Tauri / Electron

Notes

• Core engine already exists; this is wiring + UX.
• Requires the relevant ops exposed in each binding (incl. C FFI — see 0.4.0: Expose render + page metadata in the C FFI #704).