Enhance newsletter HTML generation and UI components

- Introduce helper functions `_outlook_font` and `_outlook_link` for better formatting in Outlook-compatible HTML.
- Update `create_outlook_html` to utilize new font and link functions, improving the overall appearance of the newsletter.
- Modify CSS to add styles for a new copy dialog overlay and related components.
- Refactor JavaScript to improve copy functionality and add a dialog for copying HTML content.
- Update dashboard template to enhance user instructions for copying and downloading HTML content.

app/static/css/main.css

@@ -24,3 +24,10 @@ th, td { border-bottom: 1px solid #e6ebf4; text-align: left; padding: 0.55rem; v
 .display-options legend { font-weight: 600; padding: 0 0.25rem; }
 .error { color: #b42318; background: #fef3f2; border: 1px solid #fecdca; padding: 0.6rem; border-radius: 4px; margin: 0 0 0.75rem; }
 .hint { color: #64748b; font-size: 0.92rem; margin: 0 0 0.6rem; }
+.copy-dialog-overlay { position: fixed; inset: 0; z-index: 1000; background: rgba(15, 23, 42, 0.55); display: flex; align-items: center; justify-content: center; padding: 1rem; }
+.copy-dialog { width: min(900px, 100%); max-height: 90vh; background: #fff; border-radius: 8px; padding: 1rem 1.25rem; box-shadow: 0 12px 40px rgba(0, 0, 0, 0.2); display: grid; gap: 0.75rem; }
+.copy-dialog h4 { margin: 0; color: #003366; }
+.copy-dialog-hint { margin: 0; color: #334155; }
+.copy-dialog-textarea { width: 100%; min-height: 320px; font-family: Consolas, "Courier New", monospace; font-size: 0.85rem; line-height: 1.4; border: 1px solid #cbd5e1; border-radius: 4px; padding: 0.75rem; resize: vertical; }
+.copy-dialog-actions { display: flex; gap: 0.6rem; flex-wrap: wrap; }
+.textarea-actions { display: flex; gap: 0.6rem; flex-wrap: wrap; margin-bottom: 0.6rem; }