Add color picker functionality to external calendars

- Enable clicking external calendar color icons to open color picker dropdown
- Implement backend API integration for updating external calendar colors
- Add conditional hover effects to prevent interference with color picker
- Use extremely high z-index (999999) to ensure dropdown appears above all elements
- Match existing CalDAV calendar color picker behavior and styling
- Support real-time color updates with immediate visual feedback
- Maintain color consistency across sidebar and calendar events

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

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

frontend/styles.css

@@ -281,7 +281,7 @@ body {
     border-radius: 4px;
     padding: 1rem;
     box-shadow: 0 4px 12px rgba(0,0,0,0.15);
-    z-index: 1000;
+    z-index: 999999;
     display: grid;
     grid-template-columns: repeat(4, 1fr);
     gap: 0.75rem;
@@ -3677,7 +3677,7 @@ body {
     border: 1px solid var(--glass-bg);
 }
 
-.external-calendar-info:hover {
+.external-calendar-info:hover:not(.color-picker-active) {
     background: var(--glass-bg);
     border-color: var(--glass-bg-light);
     transform: translateX(2px);