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/entities/Font/api/proxy/proxyFonts.test.ts

@@ -21,6 +21,7 @@ vi.mock('$shared/api/api', () => ({
 import { api } from '$shared/api/api';
 import { queryClient } from '$shared/api/queryClient';
 import { fontKeys } from '$shared/api/queryKeys';
+import { FontResponseError } from '../../lib/errors/errors';
 import {
     fetchFontsByIds,
     fetchProxyFontById,
@@ -86,16 +87,20 @@ describe('proxyFonts', () => {
             expect(calledUrl).toContain('offset=0');
         });
 
-        test('should throw on invalid response (missing fonts array)', async () => {
+        test('should throw FontResponseError on invalid response (missing fonts array)', async () => {
             mockApiGet({ total: 0 });
 
-            await expect(fetchProxyFonts()).rejects.toThrow('Proxy API returned invalid response');
+            await expect(fetchProxyFonts()).rejects.toSatisfy(
+                e => e instanceof FontResponseError && e.field === 'response.fonts',
+            );
         });
 
-        test('should throw on null response data', async () => {
+        test('should throw FontResponseError on null response data', async () => {
             vi.mocked(api.get).mockResolvedValueOnce({ data: null, status: 200 });
 
-            await expect(fetchProxyFonts()).rejects.toThrow('Proxy API returned invalid response');
+            await expect(fetchProxyFonts()).rejects.toSatisfy(
+                e => e instanceof FontResponseError && e.field === 'response',
+            );
         });
     });
 

src/entities/Font/api/proxy/proxyFonts.ts

@@ -15,6 +15,7 @@ import { queryClient } from '$shared/api/queryClient';
 import { fontKeys } from '$shared/api/queryKeys';
 import { buildQueryString } from '$shared/lib/utils';
 import type { QueryParams } from '$shared/lib/utils';
+import { FontResponseError } from '../../lib/errors/errors';
 import type { UnifiedFont } from '../../model/types';
 
 /**
@@ -96,11 +97,16 @@ export interface ProxyFontsParams extends QueryParams {
 /**
  * Proxy API response
  *
- * Includes pagination metadata alongside font data
+ * Includes pagination metadata alongside font data.
+ *
+ * Contract: `fonts` is always an array — never `null` or omitted, even when
+ * `total === 0`. Returning `null` on the wire is a backend regression and
+ * surfaces as FontResponseError (non-retryable) on the client.
  */
 export interface ProxyFontsResponse {
     /**
-     * List of font objects returned by the proxy
+     * List of font objects returned by the proxy.
+     * Always an array; empty when no matches.
      */
     fonts: UnifiedFont[];
 
@@ -156,8 +162,11 @@ export async function fetchProxyFonts(
 
     const response = await api.get<ProxyFontsResponse>(url);
 
-    if (!response.data || !Array.isArray(response.data.fonts)) {
-        throw new Error('Proxy API returned invalid response');
+    if (!response.data) {
+        throw new FontResponseError('response', response.data);
+    }
+    if (!Array.isArray(response.data.fonts)) {
+        throw new FontResponseError('response.fonts', response.data.fonts);
     }
 
     return response.data;

src/entities/Font/lib/errors/errors.ts

@@ -1,3 +1,5 @@
+import { NonRetryableError } from '$shared/api/queryClient';
+
 /**
  * Thrown when the network request to the proxy API fails.
  * Wraps the underlying fetch error (timeout, DNS failure, connection refused, etc.).
@@ -12,11 +14,13 @@ export class FontNetworkError extends Error {
 
 /**
  * Thrown when the proxy API returns a response with an unexpected shape.
+ * Extends NonRetryableError because schema mismatches are not transient —
+ * retrying will produce the same failure and only delay surfacing the bug.
  *
  * @property field - The name of the field that failed validation (e.g. `'response'`, `'response.fonts'`).
  * @property received - The actual value received at that field, for debugging.
  */
-export class FontResponseError extends Error {
+export class FontResponseError extends NonRetryableError {
     readonly name = 'FontResponseError';
 
     constructor(

src/entities/Font/model/store/fontCatalogStore/fontCatalogStore.svelte.ts

@@ -441,6 +441,11 @@ export class FontCatalogStore {
         try {
             response = await fetchProxyFonts(params);
         } catch (cause) {
+            // Preserve non-retryable validation errors so the query client doesn't
+            // burn the retry budget on a deterministic schema mismatch.
+            if (cause instanceof FontResponseError) {
+                throw cause;
+            }
             throw new FontNetworkError(cause);
         }
 

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;

src/shared/api/queryClient.ts

@@ -1,5 +1,15 @@
 import { QueryClient } from '@tanstack/query-core';
 
+/**
+ * Marker base class for errors that retrying will never fix — schema-validation
+ * failures, unauthorized responses, contract violations, etc.
+ *
+ * The queryClient retry handler short-circuits when it sees this; without it,
+ * a non-transient backend bug pins the UI through the full retry budget
+ * (default 3× exponential backoff ≈ 7s).
+ */
+export class NonRetryableError extends Error {}
+
 /**
  * Data remains fresh for this long after fetch. Stores that override
  * staleness (e.g. filtered queries) can use 0 to bypass.
@@ -51,7 +61,12 @@ export const queryClient = new QueryClient({
              * Refetch on mount if data is stale
              */
             refetchOnMount: true,
-            retry: QUERY_RETRY_COUNT,
+            retry: (failureCount, error) => {
+                if (error instanceof NonRetryableError) {
+                    return false;
+                }
+                return failureCount < QUERY_RETRY_COUNT;
+            },
             /**
              * Exponential backoff: 1s, 2s, 4s, 8s... capped at 30s
              */