## Context
We've gotten some requests for a better markdown editing experience in
the Explorer so this PR adds a proper live preview markdown editor,
similar UX to how you'd edit notion or obsidian documents. Opting to use
`lexical` as the main library to support this UI for its size and
composability without going too low level like prose mirror
<img width="933" height="452" alt="image"
src="https://github.com/user-attachments/assets/7161ae6c-3dff-4b36-818f-01b5e573f121"
/>
Actual file changes are just this size excluding the package-lock
<img width="150" height="39" alt="image"
src="https://github.com/user-attachments/assets/bb01f2cf-8ef7-4daf-975b-614b2d817b54"
/>
## Changes involved
This adjusts the UX for markdown cells a little
- Clicking on a markdown cell immediately allows you to edit
- Instead of the existing behaviour which requires a double click to
enter "Edit" state
- Editing doesn't eject you into a monaco editor, but rather inline
editor
- Nicer transition
- Clicking out of the cell persists the cell's content
- No more "Cancel" or "Save" buttons
## To test
- [ ] Verify that editing markdown cells are still working
- [ ] Verify that tabs do get marked with unsaved changes if you edit a
markdown cell
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
- **New Features**
- Replaced the previous Markdown editing workflow with an always-visible
inline editor.
- Added rich Markdown editing for headings, lists, links, blockquotes,
code blocks, inline code, and text formatting.
- Changes are saved automatically when leaving the editor.
- Markdown formatting shortcuts take precedence within the editor.
- **Bug Fixes**
- Avoids unnecessary saves when content has not changed.
- **Tests**
- Added coverage verifying Markdown formatting conversions remain
consistent and repeatable.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
