fix(work): make AI Work application undoable as a single editor edit

[harsh-thakkar7]

Problem

Applying an AI Work result went straight through onDocumentChange, which updates React state but never performs an editor operation — so the AI replacement had no undo boundary of its own in Monaco. As issue #300 describes, a single Undo could not restore the exact pre-Work document.

Change

• New importable module frontend/src/lib/apply-work-edit.mjs exporting applyWorkEdit(). When the mounted editor/model is available it replaces the full document with editor.executeEdits("ai-work", …) wrapped in editor.pushUndoStop() on both sides, so one Undo restores the pre-Work document and Redo reapplies the AI result without merging into the user’s previous typing.
• useAIActions.executeWork() routes the (still freshness-guarded) result through applyWorkEdit, passing editorRef.current. The edit flows through the editor change callback, so the existing state/preview synchronization path is preserved.
• Focus mode / editor-not-mounted falls back to onDocumentChange exactly as before.

Acceptance criteria (from #300)

1. Applying to the unchanged active document is functionally equivalent — covered.
2. One Undo restores the exact pre-Work content — covered.
3. Redo restores the AI-applied content — covered.
4. The replacement does not merge into the previous typing undo step — covered.
5. Per-tab model isolation untouched — no model lifecycle changes.
6. Focused regression coverage added; frontend test/lint/build green.

Tests

frontend/tests/ai-work-undo.test.mjs drives a real Monaco model: editor-vs-fallback routing, single-Undo/Redo, no merge with prior typing, consecutive AI applies each individually undoable, an unsealed prior edit not swallowed, and the no-editor fallback. Mutation-tested (dropping the opening/trailing undo stops, and forcing the fallback) — each mutation is caught.

Local: npm test 101 passed, npm run lint clean, npm run build green.

Fixes #300.

[petertzy]

Approved. This is a necessary and well-scoped fix for the AI Work undo behavior. The implementation routes the full-document replacement through Monaco with explicit undo stops, preserving the existing change callback for state and preview synchronization while retaining the no-editor fallback. The focused regression coverage verifies undo/redo, separation from prior typing, consecutive Work applications, and fallback behavior. Frontend tests and lint pass.