Improve external calendar modal UI and remove emojis

- Fix modal backdrop and centering by using proper modal-backdrop class
- Make color picker more compact (80px width instead of 100%)
- Add Outlook 365 setup instructions with step-by-step guide
- Remove calendar emojis from button and sidebar indicators for cleaner design

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

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

frontend/styles.css

@@ -3840,7 +3840,7 @@ body {
 }
 
 .create-external-calendar-button::before {
-    content: "📅";
+    content: "";
     position: absolute;
     left: 1rem;
     font-size: 0.8rem;
@@ -3953,7 +3953,7 @@ body {
 }
 
 .external-calendar-modal .form-group input[type="color"] {
-    width: 100%;
+    width: 80px;
     height: 40px;
     padding: 0;
     border: 1px solid #ced4da;