Add Tiptap slash editor

[bnz183]

Summary

• Replaces the textarea body editor with a custom Tiptap-powered rich editor and slash commands.
• Preserves Markdown/MDX as the source of truth with locked raw MDX blocks in rich mode and a source Markdown mode fallback.
• Keeps autosave, outline, preview, internal links, and publish flows working against the body string.

Test plan

• pnpm build
• pnpm test
• pnpm test:e2e
• Verify rich editing round-trips common Markdown (headings, lists, links, images)
• Verify unknown MDX JSX blocks stay intact via source mode and rich-mode placeholders
• Verify slash commands insert expected blocks

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