fix: surface list-fetch failures in paginated mode (#1998)

#1954 wired the list-load error off the aggregate managed stores only.
With `paginatedLists` on, those stores deliberately skip their all-page
walk, so their error is permanently null — while the paged stores that
actually drive the sidebar had no error state at all and `loadPage` had
a try/finally with no catch. A failing list showed an empty panel with
no alert and no Retry, and the connect-time `void loadPage(undefined)`
left an unhandled rejection.

- Give PagedTools/Prompts/ResourcesState observable error state: an
  `errorChange` event plus `getError()`, set in a new catch in
  `loadPage`, cleared on the next success and on disconnect. The
  rejection is still re-thrown, matching ManagedListState — callers'
  auth-recovery wrappers key off it to detect a 401.
- Catch the connect-time load's rejection (`.catch(() => {})`) now that
  the failure is recorded as state rather than lost.
- Select the error by mode in `usePaginatedList`, the same way `items`
  already is, and read it in App.tsx from the pagination model.
- Rename `useManagedListError` to `useListError`: both store families
  expose an identical `errorChange`, so the hook declares that one-event
  contract itself instead of importing ManagedListEventMap.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0177mPHAdECD18nLLCTwR5rh
Signed-off-by: cliffhall <cliff@futurescale.com>

core/react/usePagedPrompts.ts

@@ -7,6 +7,7 @@ import type {
 } from "../mcp/state/pagedPromptsState.js";
 import type { Prompt } from "@modelcontextprotocol/client";
 import type { TypedEventGeneric } from "../mcp/typedEventTarget.js";
+import { useListError } from "./useListError.js";
 
 export interface UsePagedPromptsResult {
   prompts: Prompt[];
@@ -14,6 +15,12 @@ export interface UsePagedPromptsResult {
   nextCursor?: string;
   /** Pages loaded since the last reset (page 1 = 1). */
   pageCount: number;
+  /**
+   * The last page load's failure, or `null` when it succeeded. In paginated
+   * mode this store is the display source, so this — not the managed
+   * store's error — is what the panel renders (#1998).
+   */
+  error: Error | null;
   loadPage: (
     cursor?: string,
     metadata?: Record<string, string>,
@@ -72,6 +79,8 @@ export function usePagedPrompts(
     };
   }, [pagedPromptsState]);
 
+  const error = useListError(pagedPromptsState);
+
   const loadPage = useCallback(
     async (
       cursor?: string,
@@ -89,5 +98,5 @@ export function usePagedPrompts(
     pagedPromptsState?.clear();
   }, [pagedPromptsState]);
 
-  return { prompts, nextCursor, pageCount, loadPage, clear };
+  return { prompts, nextCursor, pageCount, error, loadPage, clear };
 }