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/src/components/sidebar.rs

@@ -275,7 +275,7 @@ pub fn sidebar(props: &SidebarProps) -> Html {
                                                         style={format!("background-color: {}", cal.color)}
                                                     />
                                                     <span class="external-calendar-name">{&cal.name}</span>
-                                                    <span class="external-calendar-indicator">{"📅"}</span>
+                                                    <span class="external-calendar-indicator"></span>
                                                 </div>
                                                 {
                                                     if *external_context_menu_open == Some(cal.id) {