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/features/FontManagement/lib/filterBridge.svelte.ts

@@ -0,0 +1,115 @@
+import {
+    filterValidValues,
+    isValidFontCategory,
+    isValidFontProvider,
+    isValidFontSubset,
+} from '$entities/Font/lib/typeGuards';
+import type { UnifiedFontStore } from '$entities/Font/model/store';
+import { filterManager } from '$features/FilterFonts';
+import type { Property } from '$shared/lib';
+
+/**
+ * ============================================================================
+ * FILTER BRIDGE
+ * ============================================================================
+ *
+ * Bridges the UI filter state (filterManager from FilterFonts feature) with the
+ * unified font store (unifiedFontStore from Font entity).
+ *
+ * OPTIMIZATIONS (P1):
+ * - Runtime type validation using type guards
+ * - No more 'as any[]' casts - type-safe assignments
+ */
+
+// ... (comments)
+
+// ============================================================================
+// FILTER GROUP MAPPING
+// ============================================================================
+
+/**
+ * Get selected values from a filter manager group by ID
+ */
+function getSelectedFilterValues(groupId: string): string[] {
+    const group = filterManager.getGroup(groupId);
+    if (!group) return [];
+
+    return group.instance.selectedProperties.map((property: Property<string>) => property.value);
+}
+
+// ============================================================================
+// SYNC LOGIC
+// ============================================================================
+
+/**
+ * Sync filter manager state to unified font store
+ *
+ * @param store - The unified font store instance
+ */
+export function syncFilters(store: UnifiedFontStore): void {
+    const providers = getSelectedFilterValues('providers');
+    const categories = getSelectedFilterValues('categories');
+    const subsets = getSelectedFilterValues('subsets');
+
+    // Validate and filter providers
+    const validProviders = filterValidValues(providers, isValidFontProvider);
+
+    // Validate and filter categories
+    const validCategories = filterValidValues(categories, isValidFontCategory);
+
+    // Validate and filter subsets
+    const validSubsets = filterValidValues(subsets, isValidFontSubset);
+
+    // Update unified store filters with validated, type-safe values
+    store.filters = {
+        providers: validProviders,
+        categories: validCategories,
+        subsets: validSubsets,
+        searchQuery: store.filters.searchQuery,
+    };
+}
+
+// ============================================================================
+// PUBLIC API
+// ============================================================================
+
+/**
+ * Apply current filters and fetch fonts from providers
+ *
+ * @param store - The unified font store instance
+ */
+export async function applyFilters(store: UnifiedFontStore): Promise<void> {
+    await store.fetchFonts();
+}
+
+/**
+ * Reset all filters to their default state
+ *
+ * @param store - The unified font store instance
+ */
+export function resetFilters(store: UnifiedFontStore): void {
+    // Reset filter manager selections
+    filterManager.deselectAllGlobal();
+
+    // Clear unified store filters
+    store.clearFilters();
+}
+
+/**
+ * Apply current filter selections and fetch fonts
+ *
+ * @param store - The unified font store instance
+ */
+export async function applyFilterType(store: UnifiedFontStore): Promise<void> {
+    syncFilters(store);
+    await store.fetchFonts();
+}
+
+// ============================================================================
+// RE-EXPORTS FOR CONVENIENCE
+// ============================================================================
+
+/**
+ * Re-export filter manager for direct access
+ */
+export { filterManager } from '$features/FilterFonts';