feat: markdown tabs in rendered view via ```tabs fenced block

[Copilot]

No native way to author tabbed content panels inside a note. Adds a TabGroupExtension TipTap node that renders ```tabs ``` fenced code blocks as interactive tabbed panels in the WYSIWYG editor, round-tripping cleanly back to markdown.

Syntax

```tabs
## Installation
Run `npm install my-package` to get started.

## Usage
Import and call `myPackage.init()`.

## API
See the full reference below.
```

Tabs are delimited by ## headings within the fenced block. Everything before the first heading is ignored.

Changes

• packages/editor/src/tabs-extension.tsx — new TabGroupExtension TipTap Node:
  • ProseMirror plugin intercepts codeBlock[language=tabs] nodes and replaces them with a tabGroup atom rendered via ReactNodeViewRenderer
  • Exported parseTabsContent / serializeTabsContent helpers for split/join of tab bodies
  • Lightweight inline markdown-to-HTML converter for tab panel bodies (headings, bold/italic, lists, code, links, blockquotes) — no additional parser dependency
  • Heading detection is string-based (no capturing-group regex) to avoid ReDoS
• packages/editor/src/rendered-editor.tsx — registers TabGroupExtension in the editor extension list
• packages/editor/src/index.ts — re-exports the new module
• apps/web/src/styles.css — .tab-group component styles using existing CSS design tokens
• packages/editor/src/tabs-extension.test.ts — 9 unit tests for parse/serialize round-trip

• Fixes Add tabs within rendered view #72

[adam4813]

@copilot you forgot to run lint and test. Review the lint and test errors/warnings and fix them

[Copilot]

@copilot you forgot to run lint and test. Review the lint and test errors/warnings and fix them

Fixed in the latest commit. ESLint flagged 3 no-useless-escape errors in character classes (/^[*\-] / → /^[*-] /) inside tabs-extension.tsx. Lint is now clean and all 165 tests pass.