Fix recurring event series modification and UI issues

Backend fixes:
- Fix "this event only" EXDATE handling - ensure proper timezone conversion for exception dates
- Remove debug logging for cleaner production output

Frontend fixes:
- Add EXDATE timezone conversion in convert_utc_to_local function
- Fix event duplication when viewing weeks across month boundaries with deduplication logic
- Update CSS theme colors for context menus, recurrence options, and recurring edit modals

These changes ensure RFC 5545 compliance for recurring event exceptions and improve the user experience across different themes and calendar views.

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

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

frontend/styles.css

@@ -2586,7 +2586,7 @@ body {
     display: flex;
     align-items: center;
     padding: 0.75rem 1.25rem;
-    color: #374151;
+    color: var(--text-primary);
     cursor: pointer;
     transition: all 0.15s cubic-bezier(0.4, 0, 0.2, 1);
     font-size: 0.875rem;
@@ -2607,8 +2607,8 @@ body {
 }
 
 .context-menu-item:hover {
-    background: linear-gradient(135deg, #f8fafc 0%, #f1f5f9 100%);
-    color: #1f2937;
+    background: var(--background-tertiary);
+    color: var(--text-primary);
     transform: translateX(2px);
 }
 
@@ -2834,12 +2834,12 @@ body {
     font-size: 1rem;
     font-weight: 600;
     margin-bottom: 0.5rem;
-    color: #333;
+    color: var(--text-primary);
 }
 
 .recurring-option .option-description {
     font-size: 0.9rem;
-    color: #666;
+    color: var(--text-secondary);
     line-height: 1.4;
 }
 
@@ -4487,8 +4487,8 @@ body {
 .recurrence-options {
     margin-top: 1.5rem;
     padding: 1rem;
-    background: #f8f9fa;
-    border: 1px solid #e9ecef;
+    background: var(--background-secondary);
+    border: 1px solid var(--border-primary);
     border-radius: var(--border-radius-medium);
 }