fix(SEC-REVIEW-1): Surface search errors in LinkAutocomplete

Previously the catch block in searchEntries silently swallowed all
non-abort errors, showing "No entries found" when the search actually
failed. This misled users into thinking the knowledge base was empty.

- Add searchError state variable
- Set PDA-friendly error message on non-abort failures
- Clear error state on subsequent successful searches
- Render error in amber (distinct from gray "No entries found")
- Add 3 tests: error display, error clearing, abort exclusion

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/web/src/components/knowledge/LinkAutocomplete.tsx

@@ -49,6 +49,7 @@ export function LinkAutocomplete({
   const [results, setResults] = useState<SearchResult[]>([]);
   const [selectedIndex, setSelectedIndex] = useState<number>(0);
   const [isLoading, setIsLoading] = useState<boolean>(false);
+  const [searchError, setSearchError] = useState<string | null>(null);
   const dropdownRef = useRef<HTMLDivElement>(null);
   const searchTimeoutRef = useRef<NodeJS.Timeout | null>(null);
   const abortControllerRef = useRef<AbortController | null>(null);
@@ -88,6 +89,7 @@ export function LinkAutocomplete({
 
       setResults(searchResults);
       setSelectedIndex(0);
+      setSearchError(null);
     } catch (error) {
       // Ignore aborted requests - a newer search has superseded this one
       if (error instanceof DOMException && error.name === "AbortError") {
@@ -95,6 +97,7 @@ export function LinkAutocomplete({
       }
       console.error("Failed to search entries:", error);
       setResults([]);
+      setSearchError("Search unavailable — please try again");
     } finally {
       if (!signal.aborted) {
         setIsLoading(false);
@@ -371,6 +374,8 @@ export function LinkAutocomplete({
     >
       {isLoading ? (
         <div className="p-3 text-sm text-gray-500 dark:text-gray-400">Searching...</div>
+      ) : searchError ? (
+        <div className="p-3 text-sm text-amber-600 dark:text-amber-400">{searchError}</div>
       ) : results.length === 0 ? (
         <div className="p-3 text-sm text-gray-500 dark:text-gray-400">
           {state.query ? "No entries found" : "Start typing to search..."}