Implement interactive calendar color picker

Backend enhancements:
- Add calendar_path field to CalendarEvent for color mapping
- Generate consistent colors for calendars using path-based hashing
- Update CalDAV parsing to associate events with their calendar paths
- Add 16-color palette with hash-based assignment algorithm

Frontend features:
- Interactive color picker with 4x4 grid of selectable colors
- Click color swatches to open dropdown with all available colors
- Instant color changes for both sidebar and calendar events
- Persistent color preferences using local storage
- Enhanced UX with hover effects and visual feedback

Styling improvements:
- Larger 16px color swatches for better clickability
- Professional color picker dropdown with smooth animations
- Dynamic event coloring based on calendar assignment
- Improved contrast with text shadows and borders
- Click-outside-to-close functionality for better UX

Users can now personalize their calendar organization with custom colors
that persist across sessions and immediately update throughout the app.

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

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

backend/src/handlers.rs

@@ -145,8 +145,9 @@ pub async fn get_user_info(
                 None
             } else {
                 Some(CalendarInfo {
-                    path,
+                    path: path.clone(),
                     display_name,
+                    color: generate_calendar_color(&path),
                 })
             }
         }).collect();
@@ -158,6 +159,39 @@ pub async fn get_user_info(
     }))
 }
 
+// Helper function to generate a consistent color for a calendar based on its path
+fn generate_calendar_color(path: &str) -> String {
+    // Predefined set of attractive, accessible colors for calendars
+    let colors = [
+        "#3B82F6", // Blue
+        "#10B981", // Emerald
+        "#F59E0B", // Amber
+        "#EF4444", // Red
+        "#8B5CF6", // Violet
+        "#06B6D4", // Cyan
+        "#84CC16", // Lime
+        "#F97316", // Orange
+        "#EC4899", // Pink
+        "#6366F1", // Indigo
+        "#14B8A6", // Teal
+        "#F3B806", // Yellow
+        "#8B5A2B", // Brown
+        "#6B7280", // Gray
+        "#DC2626", // Red-600
+        "#7C3AED", // Violet-600
+    ];
+    
+    // Create a simple hash from the path to ensure consistent color assignment
+    let mut hash: u32 = 0;
+    for byte in path.bytes() {
+        hash = hash.wrapping_mul(31).wrapping_add(byte as u32);
+    }
+    
+    // Use the hash to select a color from our palette
+    let color_index = (hash as usize) % colors.len();
+    colors[color_index].to_string()
+}
+
 // Helper function to extract a readable calendar name from path
 fn extract_calendar_name(path: &str) -> String {
     // Extract the last meaningful part of the path