fix(editor): support heading node placeholders

[Iiviavs]

Description

Fixes an issue where placeholder text for empty heading nodes (e.g., "Heading 1", "Heading 2") was not displayed in @react-email/editor.

Root Cause

1. CSS Selector Scope: The placeholder pseudo-element rule in packages/editor/src/ui/themes/default.css was strictly targeting .tiptap p.is-empty::before.
2. React Node View DOM Structure: The Heading extension uses ReactNodeViewRenderer, which renders content wrapped inside a NodeViewWrapper <div>. Tiptap attaches is-empty and data-placeholder attributes to this outer wrapper element rather than h1–h6 tags directly.

Changes

• Updated packages/editor/src/ui/themes/default.css to target h1–h6.is-empty::before and div.is-empty[data-placeholder]::before.
• Removed the outdated TODO in packages/editor/src/email-editor/email-editor.tsx.

Testing

• Ran @react-email/editor unit tests (502 tests passing).
• Verified placeholder styling in empty heading and paragraph nodes.

---

Summary by cubic

Fixes missing placeholder text for empty heading nodes in @react-email/editor. Previously placeholders appeared only for empty paragraphs; now headings show their placeholder by expanding the CSS selector to match where Tiptap applies is-empty and data-placeholder.

Review notes

• Expand the .tiptap ...::before rule to h1–h6 and .tiptap div.is-empty[data-placeholder] to cover React NodeView wrappers.
• Remove an obsolete TODO; no runtime or API changes.
• No migration required.

^{Written for commit dbf4cd9. Summary will update on new commits.}

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