feat: CodeMirror TeX editor and split sandbox layout

[pyramation]

Summary

Replaces the sandbox's plain <textarea> with CodeMirror 6 (stex stream mode → real LaTeX highlighting, line numbers, ⌘/Ctrl+Enter to render), and turns /sandbox into a full-height split workspace: editor left, render right, each scrolling independently, plus a Hide editor toggle that gives the render the full width.

CodeMirror touches the DOM as it loads, and this site is a static export, so the editor is split in two: tex-editor.tsx is the SSR-safe entry (dynamic(..., { ssr: false })), and tex-editor-client.tsx holds the actual widget and language mode.

// tex-editor-client.tsx
const extensions = [StreamLanguage.define(stex)];
<CodeMirror value={value} onChange={onChange} height='100%' extensions={extensions} />

The example pages' existing "Edit source" textarea uses the same component (fixed h-96 there — those pages stay prose-width); CodeBlock, Render/Reset, Open in sandbox, the example <select>, #tex= deep links and the SEO/JSON-LD block are unchanged.

Layout now widens main to max-w-7xl and makes it a flex column on /sandbox only, so the workspace can fill the viewport instead of the reading measure everything else uses.

Deps pinned exactly: @uiw/react-codemirror@4.25.11, @codemirror/legacy-modes@6.5.3, @codemirror/language@6.12.4.

Verified in the browser: highlighting, example load + render, the toggle, and the example page's inline editor. Tests and the full static export pass.

Based on chore/bump-latex2js-4.4.0 (#5), so this diff includes that bump until it merges.

Link to Devin session: https://app.devin.ai/sessions/e51e8ecb94d241bd8d89e65e642c3e43
Requested by: @pyramation

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