[clbotdev]

Closes #169.

Redesigns the email editor around writing, following the design attached to the issue.

What changes

Write mode

• The subject and preview text sit above the email, with an inbox preview.
• Text is edited paragraph by paragraph:
  • Enter starts a new paragraph or list item; Backspace joins paragraphs.
  • Markdown shortcuts (#, ##, -, 1., >) turn a paragraph into a heading, list or quote.
  • Merge tags show as chips.
  • Text blocks are still stored as one markdown block each, so there are no data changes.
• A format bar (⌘B/I/K), a / block menu, a {{ merge-tag menu, and drag or arrow-key reordering.
• Clicking a block opens a compact popover (for example, label and URL for a button). Its full settings are one click away in Style mode.
• Undo and redo work across the whole document.
• A "Before you send" checklist covers the missing subject, preview text, broken links, missing alt text and social URLs, with one-click fallbacks for merge tags that may be blank.

Style mode

• Two independently scrolling cards: the canvas, and the Email / Block / Tags panel.
• Desktop, mobile, sample-data and dark-inbox previews, palette presets, and typography settings.

New blocks

• spacer, columns, social and html.
• conditional: a section shown to contacts with or without a tag. Marketing only; it renders as Liquid and fails closed.

For packages that embed the editor

• New EmailEditor props: subject, onSubjectChange, variables, senderName, theme and defaultMode.
• Blocks can define quickEdit for the popover.
• Hosts can still add their own blocks and leave out any or all of ours; component tests cover this.

API

• The new blocks are validated on write.
• conditional in transactional content is rejected with conditional_not_allowed.
• Updated: REST error docs, MCP tool text, and the editor docs.

Web

• Template and sequence-email screens use the new editor; the template title moves into the top bar.
• A broadcast's subject is saved as its sequence title, which is what the API sends.

Testing

• bun run test passes in every workspace (0 failures), and lint passes.
• New tests:
  • Unit tests for text items, markdown editing, history, checklist and inline markdown.
  • happy-dom component tests for host blocks and writing behaviour.
  • API tests for validation and real Liquid rendering of conditionals.
• Tested by hand in a browser: writing, slash menu, popovers, drag, undo, checklist fallbacks, Style previews, and the save round trip for templates and broadcasts.
• Not tested: "Send test" and a real send, because no ESP was configured locally.

Known rough edges (not in this PR)

• A new broadcast's subject starts as the template name.
• Nothing has focus when the editor opens.
• While a paragraph with a link is being edited, the raw link markdown changes how lines wrap.
• "Your sender name" shows when no ESP is set up.