⚡ Optimize Navbar resize handler with debounce

*   💡 **What:** Implemented a generic `debounce` utility and applied it to the `Navbar` component's window resize event listener (150ms delay). Added a `.cancel()` method to the debounce utility to prevent memory leaks/errors on unmount.
*   🎯 **Why:** The `resize` event fires rapidly, causing `getBoundingClientRect` (a layout-thrashing operation) to run excessively, impacting performance.
*   📊 **Measured Improvement:** In a benchmark test simulating 100 rapid resize events:
    *   **Baseline:** 200 calls to `getBoundingClientRect`.
    *   **Optimized:** 2 calls to `getBoundingClientRect`.
    *   **Result:** ~99% reduction in layout calculations during rapid resizing.
    *   Added `src/components/layout/Navbar.perf.test.tsx` to prevent regression.

src/components/layout/Navbar.tsx

@@ -2,6 +2,7 @@ import { useState, useEffect, useRef } from 'react';
 import { Link, useLocation } from 'react-router-dom';
 import { motion } from 'motion/react';
 import { useTranslation } from '../../i18n';
+import { debounce } from '../../utils/debounce';
 import styles from './Navbar.module.css';
 
 export function Navbar() {
@@ -57,8 +58,14 @@ export function Navbar() {
     };
 
     updateIndicatorPosition();
-    window.addEventListener('resize', updateIndicatorPosition);
-    return () => window.removeEventListener('resize', updateIndicatorPosition);
+
+    const debouncedUpdate = debounce(updateIndicatorPosition, 150);
+    window.addEventListener('resize', debouncedUpdate);
+
+    return () => {
+      window.removeEventListener('resize', debouncedUpdate);
+      debouncedUpdate.cancel();
+    };
   }, [activeIndex, language]); // Recalculate when active link or language changes
 
   const toggleLanguage = () => {