feat: preview ChangeSets inline

[yuyukosama2004]

What changed

• add a Tiptap/ProseMirror decoration extension for pending ChangeSet operations
• mark replace/delete targets in the live editor without changing document JSON
• render insert/replace proposals as non-editable inline widgets at stable node anchors
• keep widgets ordered semantically at shared block boundaries
• synchronize the active review batch from the ChangeSet panel into the editor
• clear resolved or hidden previews immediately
• add focused extension and panel synchronization tests

Why

R3 requires authors to see AI edits in their actual manuscript context before deciding. The review panel from #18 supplied decisions and side-by-side previews, but did not show where each proposal lands in the document.

Safety

The preview is a pure ProseMirror Decoration layer. It is excluded from editor JSON, history, autosave, versions, and canonical content. Proposal text is inserted with textContent, not HTML.

Validation

• npm run format
• npm run lint
• npm test -- --run — 69 passed
• npm run build
• focused preview/order tests after final adjustment — passed
• isolated browser E2E with all four operation types
• accepted one replacement: draft revision advanced, editor reloaded, resolved decoration disappeared, remaining pending decorations stayed visible
• browser console: no runtime errors