Enhance newsletter functionality and UI improvements

- Introduce `create_outlook_html` function for generating Outlook-compatible newsletter HTML.
- Update dashboard to include `outlook_html` in the response.
- Modify newsletter generation logic to support separate handling of Outlook HTML.
- Improve CSS styles for newsletter preview and layout adjustments.
- Add new button for downloading HTML directly from the dashboard.

app/static/css/main.css

@@ -14,7 +14,8 @@ button { width: fit-content; background: #1b5dbf; color: #fff; border: 0; border
 .copy-status { font-size: 0.92rem; color: #64748b; }
 .copy-status.success { color: #027a48; }
 .copy-status.error { color: #b42318; }
-.newsletter-preview-frame { width: 100%; min-height: 520px; border: 1px solid #dde3ef; border-radius: 6px; background: #fff; }
+.newsletter-preview-frame { width: 100%; min-height: 720px; border: 1px solid #D7DEE8; border-radius: 6px; background: #EEF2F7; box-shadow: 0 2px 8px rgba(0, 51, 102, 0.08); }
+.card h3 { color: #003366; margin-top: 0; }
 .hidden-source { display: none; }
 table { width: 100%; border-collapse: collapse; }
 th, td { border-bottom: 1px solid #e6ebf4; text-align: left; padding: 0.55rem; vertical-align: top; }