[FEATURE] Desktop file viewer: allow editing `.md` files in place, as code files already allow

What's the problem?

In the desktop app's file viewer, opening a code file gives an editable buffer with a save affordance — you can type a correction and save it. Opening a .md file gives a rendered preview with no way to edit. The capability is gated by file type.

That gate happens to fall along the line between code and prose, which makes it a bigger deal than it first looks. A large and growing share of Claude Code users are not developers — people writing specs, plans, notes and documentation, often with a plugin or workflow whose primary artifacts are markdown. For those users, every file they own is on the read-only side of the gate, while the files they never touch are editable.

The asymmetry also undercuts a common review pattern: Claude writes text to a file and links it, the user opens it to approve. For a .py that review can end in a one-word fix. For a .md the user has to come back to chat and ask Claude to make the edit — a round trip, and a token cost, for a typo.

What would you like?

Editing for .md in the file viewer, with the same save affordance code files have. A source/preview toggle would be the natural shape — keep the rendered view as the default, let the user switch to source, edit, and save.

Environment

Windows 11, Claude Code desktop app 2.1.219.