Fix calendar management modal color picker issues

- Fix z-index issue by creating separate CSS classes for inline vs dropdown color pickers
- Unify CalDAV and external calendar color pickers to use same grid interface
- Improve color picker styling with 4x4 grid layout for 16 colors
- Enhance color option appearance with proper border centering and sizing
- Replace native HTML color input with consistent predefined color grid
- Add visual improvements: larger swatches, better hover effects, checkmark selection

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

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

frontend/src/components/calendar_list_item.rs

@@ -55,7 +55,7 @@ pub fn calendar_list_item(props: &CalendarListItemProps) -> Html {
                     {
                         if props.color_picker_open {
                             html! {
-                                <div class="color-picker">
+                                <div class="color-picker-dropdown">
                                     {
                                         props.available_colors.iter().map(|color| {
                                             let color_str = color.clone();