Improve agent parsing, split diff UX, and chunk-level review flow

[yjr24300180030]

Summary

• Hardened agent reply parsing for wrapped/fenced/json-like responses.
• Improved split diff readability with hunk grouping and side-by-side alignment.
• Added per-change accept/reject flow and diff-to-editor cursor linkage.
• Added guard against partial suggestion replacing the whole document.
• Restored undo/redo controls and disabled repeated send while agent is busy.

Validation

• npm --workspace apps/frontend run build
• node --check apps/backend/src/routes/agent.js
• node --check apps/backend/src/services/agentService.js
• node --check apps/backend/src/services/compileService.js