[Proposed Feature] README Editor in Browser

Description of Feature:

Add an in-browser markdown editor for README files on dataset pages. Users with write access see an "Edit" button next to the README that opens a split-pane editor with live preview. Saving writes README.md straight back to the repo's bucket with no local edit + re-uploading.

v1 scope:

• Edit button, gated on write permission
• Live preview matching the existing README render
• Save commits via the existing write path
• Cancel/discard without saving

Out of scope: editing other object types, image uploads, multi-file edits.

What value is this feature adding to Source Cooperative?

The README is the landing page for every dataset. It's how people know what the data is and how to use it. Right now updating it means editing locally and re-uploading, so docs go stale. Removing that friction gets maintainers to actually keep docs current, which directly improves discoverability and trust across the catalog.

This is especially relevant for READMEs that are created by AI agents and tools like Portolan. They need heavy editing.

It also builds on #450 (README download fix): instead of treating the README as a static file to view/download, this treats it as living documentation maintainers manage in place.

continuation of issue 450

Start by locating the existing README rendering, permission check, and write path referenced in the issue. Trace how a dataset README is displayed and saved, then verify that the v1 flow supports permission-gated editing, live preview, save, and discard without affecting other object types.