⚡ 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/utils/debounce.test.ts

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+import { describe, it, expect, vi } from 'vitest';
+import { debounce } from './debounce';
+
+describe('debounce', () => {
+  it('calls the function after the wait time', () => {
+    vi.useFakeTimers();
+    const func = vi.fn();
+    const debounced = debounce(func, 100);
+
+    debounced();
+    expect(func).not.toHaveBeenCalled();
+
+    vi.advanceTimersByTime(100);
+    expect(func).toHaveBeenCalledTimes(1);
+  });
+
+  it('cancels the pending execution', () => {
+    vi.useFakeTimers();
+    const func = vi.fn();
+    const debounced = debounce(func, 100);
+
+    debounced();
+    expect(func).not.toHaveBeenCalled();
+
+    debounced.cancel();
+
+    vi.advanceTimersByTime(100);
+    expect(func).not.toHaveBeenCalled();
+  });
+});

src/utils/debounce.ts

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+export interface DebouncedFunction<F extends (...args: any[]) => any> {
+  (...args: Parameters<F>): void;
+  cancel: () => void;
+}
+
+export function debounce<F extends (...args: any[]) => any>(
+  func: F,
+  wait: number
+): DebouncedFunction<F> {
+  let timeout: ReturnType<typeof setTimeout> | null = null;
+
+  const debounced = function (...args: Parameters<F>) {
+    if (timeout) {
+      clearTimeout(timeout);
+    }
+    timeout = setTimeout(() => {
+      func(...args);
+      timeout = null;
+    }, wait);
+  };
+
+  debounced.cancel = () => {
+    if (timeout) {
+      clearTimeout(timeout);
+      timeout = null;
+    }
+  };
+
+  return debounced as DebouncedFunction<F>;
+}