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/styles.css

@@ -260,7 +260,20 @@ body {
     transform: scale(1.1);
 }
 
+/* Inline color picker for forms */
 .color-picker {
+    background: white;
+    border-radius: 4px;
+    padding: 1rem;
+    border: 1px solid #e5e7eb;
+    display: grid;
+    grid-template-columns: repeat(4, 1fr);
+    gap: 0.75rem;
+    max-width: 220px;
+}
+
+/* Dropdown color picker for sidebar calendars */
+.color-picker-dropdown {
     position: absolute;
     top: 100%;
     left: 0;
@@ -277,23 +290,42 @@ body {
 }
 
 .color-option {
-    width: 20px;
-    height: 20px;
-    border-radius: 50%;
-    border: 2px solid rgba(0,0,0,0.1);
+    width: 38px;
+    height: 38px;
+    border-radius: 6px;
+    border: 3px solid rgba(0,0,0,0.15);
     cursor: pointer;
-    transition: all 0.2s;
+    transition: all 0.15s cubic-bezier(0.4, 0, 0.2, 1);
+    position: relative;
+    box-shadow: 0 1px 2px rgba(0,0,0,0.08);
+    box-sizing: border-box;
 }
 
 .color-option:hover {
-    transform: scale(1.2);
-    border-color: rgba(0,0,0,0.3);
+    transform: translateY(-2px);
+    border-color: rgba(0,0,0,0.2);
+    box-shadow: 0 4px 8px rgba(0,0,0,0.15);
 }
 
 .color-option.selected {
-    border-color: #333;
+    border-color: #2563eb;
     border-width: 3px;
-    transform: scale(1.1);
+    transform: translateY(-2px) scale(1.05);
+    box-shadow: 0 0 0 1px rgba(37, 99, 235, 0.3),
+                0 4px 8px rgba(0,0,0,0.15);
+}
+
+.color-option.selected::after {
+    content: '✓';
+    position: absolute;
+    top: 50%;
+    left: 50%;
+    transform: translate(-50%, -50%);
+    color: white;
+    font-size: 16px;
+    font-weight: bold;
+    text-shadow: 0 1px 3px rgba(0,0,0,0.7);
+    z-index: 1;
 }
 
 .color-picker-overlay {