fix: Improve rich text formatting in emails and add button hints

Fix highlighting display in emailed notes and add clear formatting hints to email/download buttons.

Changes:
- Added proper HTML/CSS structure to email template for rich text support
- Added CSS styles for mark (highlight), strong, em, u, s, headings, and lists
- Highlight now renders with yellow background (#fef08a) in emails
- All rich text formatting now properly displays in email clients
- Added formatting hints to buttons: "Email Notes (Formatting included)" and "Download Notes (No formatting)"
- Button hints use smaller text with opacity for subtle visual hierarchy

Email template improvements:
- Proper DOCTYPE and HTML structure
- Style block in head for rich text elements
- Removed white-space: pre-wrap from notes div to allow HTML rendering
- Maintained all existing sermon content styling

This ensures users understand that:
- Email preserves all rich text formatting (bold, italic, highlights, lists, etc.)
- Download converts to plain text for universal compatibility

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

pages/[slug].vue

@@ -135,13 +135,19 @@
                   class="flex-1 px-4 py-2 bg-blue-600 text-white rounded-md hover:bg-blue-700 disabled:bg-blue-400 disabled:cursor-not-allowed font-medium transition-colors"
                 >
                   <span v-if="emailStatus === 'sending'">Sending...</span>
-                  <span v-else>📧 Email Notes</span>
+                  <span v-else class="flex flex-col items-center">
+                    <span>📧 Email Notes</span>
+                    <span class="text-xs opacity-80 mt-0.5">(Formatting included)</span>
+                  </span>
                 </button>
                 <button
                   @click="downloadNotes"
                   class="flex-1 px-4 py-2 bg-green-600 text-white rounded-md hover:bg-green-700 font-medium transition-colors"
                 >
-                  📥 Download Notes
+                  <span class="flex flex-col items-center">
+                    <span>📥 Download Notes</span>
+                    <span class="text-xs opacity-80 mt-0.5">(No formatting)</span>
+                  </span>
                 </button>
               </div>
               <p v-if="emailStatus === 'success'" class="mt-2 text-sm text-green-600">