Add VS Code CSS validation support (#5893)

## Summary

Adds VS Code custom CSS data for proper validation of non-standard CSS
properties and Tailwind v4 directives.

## Changes

- **What**: Custom CSS data files for VS Code CSS language service
validation
  - `app-region` - Electron draggable regions  
  - `speak` - Deprecated aural stylesheet property
  - `@custom-variant` - Tailwind v4 custom variant definitions
  - `@utility` - Tailwind v4 custom utility definitions
- Fixes broken documentation links in existing Tailwind directive
references

## Review Focus

Documentation links verified against current Tailwind CSS and Electron
documentation.

## Screenshots

Current: Yellow squigglies

<img width="338" height="179" alt="image"
src="https://github.com/user-attachments/assets/652040f5-8e0b-486b-95f6-2fa8f9bf9ba7"
/><img width="499" height="180" alt="image"
src="https://github.com/user-attachments/assets/43d16210-7fbf-4ef8-b0e1-9a16e59d1d85"
/>

Proposed: Satisfying lack of warnings

<img width="173" height="62" alt="image"
src="https://github.com/user-attachments/assets/25f1c1c4-22b7-483b-9848-3030a3c0dc86"
/>

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-5893-Add-VS-Code-CSS-validation-support-2806d73d3650813fb5f1e176360c5b7e)
by [Unito](https://www.unito.io)

---------

Consequences of `@apply` usage may include but are not limited to:
- A strong talking-to
- Receipt of a corrective directive missive
- Upsetting @DrJKL

Co-authored-by: Alexander Brown <drjkl@comfy.org>

.gitignore

@@ -31,6 +31,7 @@ CLAUDE.local.md
 *.code-workspace
 !.vscode/extensions.json
 !.vscode/tailwind.json
+!.vscode/custom-css.json
 !.vscode/settings.json.default
 !.vscode/launch.json.default
 .idea