[FEATURE] Desktop App: Add live Markdown preview in Files panel

Problem Statement

When Claude edits or generates .md files, the Files panel displays raw Markdown text with no rendered preview. There is no way to see formatted output (headings, tables, code blocks) without opening a separate external tool. This breaks the workflow for documentation and design doc authoring, which is a common use case for Claude Code.

Proposed Solution

The Files panel should render .md files as formatted HTML and update in real-time as file content changes — similar to VS Code's built-in Markdown preview (⇧⌘V). Ideally: a toggle between raw and rendered view, with live refresh on file save or change.

Alternative Solutions

Currently using a separate app (e.g., Typora / Marked 2) to watch and preview the file alongside the Desktop App — requires switching between two windows constantly.

Priority

High — Significant impact on productivity

Feature Category

File operations

Use Case Example

1. Ask Claude to generate or revise a design doc (design.md)
2. Want to review the formatted output (tables, headings, code blocks) while Claude is writing
3. Currently must open a separate Markdown viewer and alt-tab between windows
4. With live preview in Files panel, the entire workflow stays inside Claude Code Desktop App