feat: add embed block node for iframes and JSX components in blog editor

[devin-ai-integration]

feat: add embed block node for iframes and JSX components in blog editor

Summary

Adds a new "Embed Block" node to the blog editor, inspired by Vrite's Element block pattern. This is an opaque atom node that stores arbitrary iframe, JSX, or HTML code as a raw string — allowing blog authors to embed <iframe>, <Callout>, <CtaCard>, or any other component markup directly in the editor.

New files:

• packages/tiptap/src/shared/embed-block.ts — base tiptap Node definition with paste detection (looksLikeEmbedCode), parseHTML/renderHTML, and markdown serialization hooks
• apps/web/src/components/admin/blog-editor/embed-block.tsx — React node view with edit mode (textarea for pasting code) and preview mode (shows code + detected type label)

Modified files:

• Blog editor index.tsx — registers EmbedBlockNode extension
• toolbar.tsx — adds <CodeXml> toolbar button to insert an embed block
• packages/tiptap/src/shared/index.ts — re-exports the new module

Behavior:

• Toolbar button inserts an empty embed block (opens in edit mode)
• Pasting iframe/JSX/HTML code auto-creates an embed block via handlePaste plugin
• Cmd/Ctrl+Enter saves, Escape cancels, Backspace on empty deletes the node
• Preview mode shows a type label (e.g. "iframe — youtube.com", "JSX Component — <Callout />") with hover-to-edit pencil button

Review & Testing Checklist for Human

• Paste handler greediness: looksLikeEmbedCode matches any <Tag>...</Tag> or <iframe ...> pattern. Pasting normal HTML (e.g. <div>hello</div>) will be intercepted. Verify this doesn't break normal paste workflows. Also check interaction with the existing ClipNode YouTube paste handler — plugin ordering determines which handler fires first.
• Markdown roundtrip: parseMarkdown unconditionally returns an embedBlock for any token routed to it (the looksLikeEmbedCode guard was removed to satisfy the MarkdownParseResult type). Load existing blog posts containing JSX (e.g. <Callout>, <Clip>) and verify they survive a full save → reload cycle without data loss or unexpected embed block creation.
• Manual smoke test: This was not tested in a browser — only CI (typecheck + fmt) passes. Open the blog editor, insert an embed block via toolbar, paste an <iframe src="..."> and a JSX snippet like <Callout type="note">text</Callout>, save, and verify the markdown output is correct.
• Edge cases in regex detection: Test with multi-line JSX, deeply nested components, and self-closing tags with unusual attribute formatting. The [\s\S]* in JSX_BLOCK_REGEX could be slow on very large clipboard payloads.

Notes

• CI passes: both ci (typecheck) and fmt (dprint) checks are green.
• navigator.platform (used for keyboard shortcut hint display) is deprecated but functional; non-blocking.
• Requested by @goranmoomin
• Devin session

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