feat: implement P0/P1 performance and code quality optimizations

P0 Performance Optimizations:
- Add debounced search (300ms) to reduce re-renders during typing
- Implement single-pass filter function for O(n) complexity
- Add TanStack Query cancellation before new requests

P1 Code Quality Optimizations:
- Add runtime type guards for filter validation
- Implement two derived values (filteredFonts + sortedFilteredFonts)
- Remove all 'as any[]' casts from filter bridge
- Add fast-path for default sorting (skip unnecessary operations)

New Utilities:
- debounce utility with 4 tests (all pass)
- filterUtils with 15 tests (all pass)
- typeGuards with 20 tests (all pass)
- Total: 39 new tests

Modified Files:
- unifiedFontStore.svelte.ts: Add debouncing, use filter/sort utilities
- filterBridge.svelte.ts: Type-safe validation with type guards
- unifiedFontStore.test.ts: Fix pre-existing bugs (missing async, duplicate imports)

Code Quality:
- 0 linting warnings/errors (oxlint)
- FSD compliant architecture (entity lib layer)
- Backward compatible store API

src/shared/lib/utils/debounce/debounce.ts

@@ -0,0 +1,43 @@
+/**
+ * ============================================================================
+ * DEBOUNCE UTILITY
+ * ============================================================================
+ *
+ * Creates a debounced function that delays execution until after wait milliseconds
+ * have elapsed since the last time it was invoked.
+ *
+ * @example
+ * ```typescript
+ * const debouncedSearch = debounce((query: string) => {
+ *     console.log('Searching for:', query);
+ * }, 300);
+ *
+ * debouncedSearch('hello');
+ * debouncedSearch('hello world'); // Only this will execute after 300ms
+ * ```
+ */
+
+/**
+ * Creates a debounced version of a function
+ *
+ * @param fn - The function to debounce
+ * @param wait - The delay in milliseconds
+ * @returns A debounced function that will execute after the specified delay
+ */
+export function debounce<T extends (...args: any[]) => any>(
+    fn: T,
+    wait: number,
+): (...args: Parameters<T>) => void {
+    let timeoutId: ReturnType<typeof setTimeout> | null = null;
+
+    return (...args: Parameters<T>) => {
+        if (timeoutId) {
+            clearTimeout(timeoutId);
+        }
+
+        timeoutId = setTimeout(() => {
+            fn(...args);
+            timeoutId = null;
+        }, wait);
+    };
+}