Fix render_markdown element

[Anthirian]

Change Summary

This fixes markdown rendering and applies GitHub style CSS.

Motivation and details

The render_markdown element didn't properly render all markdown it would receive from a variable (see #388). This was caused by joining multiple lines with \n characters. After solving this issue, I have styled the rendered markdown according to GitHub's style.

Note that OpenAI Codex was used with GPT 5.5 xhigh to assist during development of these changes. I have manually tested and confirmed the changes are good both in light mode and dark mode.

Before

After

Tasks

• Updated the documentation in docs/ (if any application behavior has changed)
• I am happy for maintainers to push small adjustments to this PR, to speed up the review cycle

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[Anthirian]

I have read the CLA Document and I hereby sign the CLA

[greptile-apps]

Greptile Summary

Fixes the root cause of issue #388 by changing join("\ ") to join("") in the Rust stdout capture (since read_line already includes the trailing newline), and replaces dangerouslySetInnerHTML with a custom HTML→React converter for proper GFM rendering with syntax highlighting and GitHub-style CSS.

Confidence Score: 4/5

Safe to merge; one minor cosmetic issue with trailing newlines in highlighted code blocks

Only P2 findings — the trailing-newline issue causes a cosmetic empty line in syntax-highlighted blocks but no functional breakage

src/components/runbooks/editor/components/Markdown.tsx — trailing newline trim on code block content

Important Files Changed

Filename	Overview
src/components/runbooks/editor/components/Markdown.tsx	Replaces dangerouslySetInnerHTML with a custom HTML→React converter; adds syntax highlighting via prism-react-renderer; minor trailing-newline cosmetic issue in highlighted code blocks
crates/atuin-desktop-runtime/src/blocks/script.rs	Correct fix: join("") instead of join("\n") since read_line already includes the trailing newline; stdout() method updated consistently; new unit test confirms behavior
src/styles.css	Comprehensive GitHub-style CSS added for .markdown-content with light/dark variants; all: revert intentionally scoped to container only
src/components/runbooks/editor/components/Markdown.test.tsx	New test file covering language normalization, syntax highlighting, and GFM structures (tables, task lists, footnotes)
src/components/runbooks/editor/blocks/MarkdownRender/index.test.ts	Added required vi.mock stubs for store and Tauri shell to unblock the existing test suite
markdown-render-fixture.md	Manual test fixture covering the full GFM surface area; helpful for visual regression testing
docs/docs/blocks/executable/markdown-render.md	Documentation updated to reflect GFM styling, new supported elements (footnotes, task lists, alignment), and GitHub-like presentation

_{Reviews (1): Last reviewed commit: "feat(markdown_render): add syntax highli..." | Re-trigger Greptile}

[greptile-apps]

Trailing newline produces empty last line in highlighted blocks

micromark emits <code class="language-ts">const x = 1;\n</code> — the text content always ends with \n. Passing that directly to Highlight causes prism-react-renderer to emit a final empty token line, adding a blank row at the bottom of every highlighted code block.

Suggested change

	<MarkdownCodeBlock
	code={firstChild.textContent?.replace(/\n$/, "") ?? ""}