Add a plain-text part to every send and a working "view in browser" link #35

Two related deliverability gaps:

1. No plain-text part. sendMail (apps/api/src/mail/send.ts) sends only html. HTML-only bulk mail scores worse with spam filters and is unreadable in text-only clients.
2. "View in browser" has nothing behind it. It appears in packages/email-editor/src/lib/default-email.ts:78 as plain text, with no link and no hosted page.

Proposal

• Generate text alongside html for every broadcast, sequence and transactional send, unless the caller passes its own text.
  • Links become text (url), list items get • , style/script/head and the hidden preheader are dropped, and blank lines are collapsed.
  • Add a "Read online: URL" line and the footer (address and unsubscribe URL) in text form.
  • https://github.com/marclou/mailcheap uses a small hand-written htmlToText; the html-to-text package is also an option.
• Add a {{view_in_browser_url}} merge tag and a hosted, signed per-email web view. If the public archive lands, public broadcasts can link to it instead.
• Optionally show the plain-text version in the preview.