fix: namespace request-handler cache per handler and stop persist retries in demo mode

The in-memory request handler cache introduced in #6075 keyed entries by
JSON.stringify(input) in a Map shared across all handlers. Any two handlers
querying the same integration with equal options (e.g. {}) collided and read
each other's data — with the mock integration serving every widget kind, all
demo widgets received whichever handler's data was cached first (beszel
getting health-monitoring payloads, etc.). Cache keys are now namespaced per
handler definition.

Also disable the query-cache persister after the first FORBIDDEN response so
demo mode doesn't retry a mutation that is always rejected.

apps/nextjs/src/app/[locale]/_client-providers/query-cache-persister.ts

@@ -1,22 +1,39 @@
 import { createAsyncStoragePersister } from "@tanstack/query-async-storage-persister";
+import { TRPCClientError } from "@trpc/client";
 import superjson from "superjson";
 
 import { fetchApi } from "@homarr/api/client";
 import { getActiveQueryCacheBoardId, queryCacheStoragePrefix } from "@homarr/api/query-cache";
 
+// Set once the server rejects a persist mutation as read-only (demo mode),
+// so we stop retrying a mutation that can never succeed.
+let persistDisabled = false;
+
+const isReadOnlyError = (error: unknown) => error instanceof TRPCClientError && error.data?.code === "FORBIDDEN";
+
 const queryCacheStorage = {
   getItem: (_key: string) => null as string | null,
   async setItem(_key: string, value: string) {
+    if (persistDisabled) return;
     const boardId = getActiveQueryCacheBoardId();
     if (!boardId) return;
     await fetchApi.queryCache.setItem.mutate({ boardId, value }).catch((error) => {
+      if (isReadOnlyError(error)) {
+        persistDisabled = true;
+        return;
+      }
       console.warn("[query-cache] persist failed", error);
     });
   },
   async removeItem(_key: string) {
+    if (persistDisabled) return;
     const boardId = getActiveQueryCacheBoardId();
     if (!boardId) return;
     await fetchApi.queryCache.removeItem.mutate({ boardId }).catch((error) => {
+      if (isReadOnlyError(error)) {
+        persistDisabled = true;
+        return;
+      }
       console.warn("[query-cache] remove failed", error);
     });
   },