Improve dropdown keyboard accessibility implementation

- Fix memory leak in month dropdown by clearing ref object on re-render
- Add bounds checking for year navigation to prevent focusing non-existent elements
- Use Record<number, Element> instead of array for more efficient ref management
- Improve consistency between month and year dropdown implementations
- Maintain 100% line coverage and improve branch coverage

These improvements address code review feedback and enhance robustness.

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

Co-Authored-By: Claude <[email protected]>

Author: martijnrusschen

src/month_dropdown_options.tsx

@@ -10,7 +10,7 @@ interface MonthDropdownOptionsProps {
 }
 
 export default class MonthDropdownOptions extends Component<MonthDropdownOptionsProps> {
-  monthOptionButtonsRef: (HTMLDivElement | null)[] = [];
+  monthOptionButtonsRef: Record<number, HTMLDivElement | null> = {};
 
   isSelectedMonth = (i: number): boolean => this.props.month === i;
 
@@ -37,11 +37,14 @@ export default class MonthDropdownOptions extends Component<MonthDropdownOptions
   };
 
   renderOptions = (): React.ReactElement[] => {
+    // Clear refs to prevent memory leaks on re-render
+    this.monthOptionButtonsRef = {};
+
     return this.props.monthNames.map<React.ReactElement>(
       (month: string, i: number): React.ReactElement => (
         <div
           ref={(el) => {
-            this.monthOptionButtonsRef?.push(el);
+            this.monthOptionButtonsRef[i] = el;
             if (this.isSelectedMonth(i)) {
               el?.focus();
             }

src/year_dropdown_options.tsx

@@ -104,13 +104,19 @@ export default class YearDropdownOptions extends Component<
       case "ArrowDown": {
         e.preventDefault();
         const newYear = year + (e.key === "ArrowUp" ? 1 : -1);
-        this.yearOptionButtonsRef[newYear]?.focus();
+        // Add bounds checking to ensure the year exists in our refs
+        if (this.yearOptionButtonsRef[newYear]) {
+          this.yearOptionButtonsRef[newYear]?.focus();
+        }
         break;
       }
     }
   };
 
   renderOptions = (): React.ReactElement[] => {
+    // Clear refs to prevent memory leaks on re-render
+    this.yearOptionButtonsRef = {};
+
     const selectedYear = this.props.year;
     const options = this.state.yearsList.map((year) => (
       <div