Add weekday selection for weekly recurrence and fix RRULE generation

- Add weekday selection UI for weekly recurring events with checkboxes
- Implement BYDAY parameter generation in RRULE based on selected days
- Fix missing RRULE generation in iCalendar output
- Convert reminder durations to proper EventReminder structs
- Add responsive CSS styling for weekday selection interface

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

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

styles.css

@@ -1271,6 +1271,54 @@ body {
     box-shadow: 0 4px 12px rgba(102, 126, 234, 0.3);
 }
 
+/* Weekday selection styles */
+.weekday-selection {
+    display: flex;
+    gap: 0.5rem;
+    flex-wrap: wrap;
+    margin-top: 0.5rem;
+}
+
+.weekday-checkbox {
+    display: flex;
+    align-items: center;
+    cursor: pointer;
+    padding: 0.5rem 0.75rem;
+    border: 1px solid #ced4da;
+    border-radius: 6px;
+    background: white;
+    transition: all 0.2s ease;
+    user-select: none;
+    min-width: 3rem;
+    justify-content: center;
+}
+
+.weekday-checkbox:hover {
+    border-color: #667eea;
+    background: #f8f9ff;
+}
+
+.weekday-checkbox input[type="checkbox"] {
+    display: none;
+}
+
+.weekday-checkbox input[type="checkbox"]:checked + .weekday-label {
+    color: white;
+}
+
+.weekday-checkbox:has(input[type="checkbox"]:checked) {
+    background: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
+    border-color: #667eea;
+    color: white;
+}
+
+.weekday-label {
+    font-size: 0.85rem;
+    font-weight: 500;
+    color: #495057;
+    transition: color 0.2s ease;
+}
+
 /* Mobile adjustments for event creation modal */
 @media (max-width: 768px) {
     .create-event-modal .form-row {
@@ -1287,4 +1335,13 @@ body {
     .create-event-modal .btn {
         width: 100%;
     }
+    
+    .weekday-selection {
+        gap: 0.25rem;
+    }
+    
+    .weekday-checkbox {
+        min-width: 2.5rem;
+        padding: 0.4rem 0.6rem;
+    }
 }