Migrate email template builder to TipTap-based rich text editor

[chamals3n4]

Problem

The email template creation UI (/templates/new?type=email) currently uses a rigid
"add block" pattern — clicking Heading, Text, or Button appends a separate
fixed card to the page. This makes editing feel clunky:

• No way to reorder blocks (drag & drop)
• No inline rich text formatting (bold, links, lists, etc.) inside the Text block
• Each block is a disconnected UI card rather than a continuous, WYSIWYG document
• Doesn't visually resemble the actual email that will be sent, despite the live
  preview panel on the right

Proposed Solution

Replace the current block-based builder with a TipTap-based rich text editor, since
TipTap is already used in the project for job description writing. Reusing it here would let us:

• Edit the whole email body as one continuous rich-text document
• Support inline formatting (bold/italic/links/lists) natively
• Insert "Button" and variable chips ({{candidate_name}}, {{job_title}}, etc.) as
  custom TipTap nodes/marks instead of separate fixed cards
• Get block reordering via drag-and-drop more naturally (TipTap supports node dragging
  with the right extensions)
• Keep the live preview panel in sync more easily by rendering TipTap's JSON/HTML output
  directly into the preview iframe

Scope / Things to Figure Out

• Whether "Button" and "Heading" become custom TipTap node types or stay as
  separate inline elements
• How {{variable}} chips are represented in the TipTap schema (custom mention-style node
  is probably cleanest, so they can't be broken by accidental text edits)