fix(api): mark schema-validation errors as non-retryable

The proxy returned `{fonts: null, total: 0}` for empty results, which
fetchProxyFonts surfaced as a generic Error. fontCatalogStore wrapped it
as FontNetworkError, and TanStack retried 3× with exponential backoff —
pinning the loading skeleton for ~7s before settling on an empty list.

Schema mismatches are deterministic; retrying only delays surfacing the
contract violation.

- shared/api/queryClient: introduce NonRetryableError marker class.
  The default retry handler short-circuits when it sees this so any
  store using the shared client gets fail-fast behavior for free.
- entities/Font/lib/errors: FontResponseError extends NonRetryableError.
- entities/Font/api/proxy/proxyFonts: throw FontResponseError (was a
  bare Error). Document that ProxyFontsResponse.fonts is always an array.
- entities/Font/.../fontCatalogStore.fetchPage: preserve a
  FontResponseError raised lower in the stack instead of re-wrapping
  it as FontNetworkError.
- features/FilterAndSortFonts/api/filters: throw NonRetryableError on
  invalid filters payloads and document the array-never-null contract.

src/features/FilterAndSortFonts/api/filters/filters.ts

@@ -9,6 +9,7 @@
 
 import { api } from '$shared/api/api';
 import { API_ENDPOINTS } from '$shared/api/endpoints';
+import { NonRetryableError } from '$shared/api/queryClient';
 
 const PROXY_API_URL = API_ENDPOINTS.filters;
 
@@ -37,7 +38,8 @@ export interface FilterMetadata {
     type: 'enum' | 'string' | 'array';
 
     /**
-     * Available filter options
+     * Available filter options.
+     * Always an array; empty when the group has no options.
      */
     options: FilterOption[];
 }
@@ -68,11 +70,16 @@ export interface FilterOption {
 }
 
 /**
- * Proxy filters API response
+ * Proxy filters API response.
+ *
+ * Contract: `filters` (and each nested `options`) is always an array — never
+ * `null` or omitted. Wire-level `null` here is a backend regression and
+ * surfaces as a non-retryable error on the client.
  */
 export interface ProxyFiltersResponse {
     /**
-     * Array of filter metadata
+     * Array of filter metadata.
+     * Always an array; empty when no filter groups are configured.
      */
     filters: FilterMetadata[];
 }
@@ -99,7 +106,7 @@ export async function fetchProxyFilters(): Promise<FilterMetadata[]> {
     const response = await api.get<FilterMetadata[]>(PROXY_API_URL);
 
     if (!response.data || !Array.isArray(response.data)) {
-        throw new Error('Proxy API returned invalid response');
+        throw new NonRetryableError('Proxy API returned invalid filters response');
     }
 
     return response.data;