Add a live preview markdown editor for explorer (#50717)

## 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 -->

apps/studio/package.json

@@ -53,6 +53,12 @@
     "@hcaptcha/react-hcaptcha": "^1.12.0",
     "@heroicons/react": "^2.1.3",
     "@hookform/resolvers": "^3.1.1",
+    "@lexical/code": "0.51.0",
+    "@lexical/link": "0.51.0",
+    "@lexical/list": "0.51.0",
+    "@lexical/markdown": "0.51.0",
+    "@lexical/react": "0.51.0",
+    "@lexical/rich-text": "0.51.0",
     "@mjackson/multipart-parser": "^0.10.1",
     "@modelcontextprotocol/sdk": "^1.29.0",
     "@modelcontextprotocol/server": "^2.0.0",
@@ -116,6 +122,7 @@
     "idb": "^8.0.2",
     "ip-num": "^1.5.1",
     "json-logic-js": "^2.0.2",
+    "lexical": "0.51.0",
     "libpg-query": "17.6.0",
     "lodash": "catalog:",
     "lodash-es": "catalog:",
@@ -168,6 +175,7 @@
     "@faker-js/faker": "^9.9.0",
     "@graphql-codegen/cli": "5.0.5",
     "@graphql-typed-document-node/core": "^3.2.0",
+    "@lexical/headless": "0.51.0",
     "@tailwindcss/postcss": "^4.2.4",
     "@tailwindcss/vite": "4.2.4",
     "@tanstack/devtools-vite": "^0.6.0",