NL-UI-02: Bring the sent email HTML into visual parity with the live archive page

Classic theme already used the brand palette; the gaps were structural. - Declare a font. The email had no font-family at all, so Outlook rendered Times New Roman while the live page is Geist. Geist via <link> where honoured, system sans fallback, MSO conditional pins Outlook to Arial. Applied inline on wrapper cells and every <a>/button. - Render the subject as a heading under the logo, mirroring the live h1. - Rebuild the footer as a band matching SiteFooter: newsletter.org logo, tagline, opt-out line, © legal. Footer text contrast raised from 2.6:1 to 5.2:1 on classic/cards. - Admin /admin/issues/{slug} preview now shows the HTML as sent instead of whitewashing the canvas; removed adminPreviewHtml (sole caller). {{unsubscribe_url}} stays the literal placeholder in the footer href. No changes to lib/ses.ts, cron/send, or the table layout.