fix(#M5-QA): address security findings from code review

Fixes 2 important-level security issues identified in M5 QA:

1. XSS Protection (SearchResults.tsx):
   - Add DOMPurify sanitization for search result snippets
   - Configure to allow only <mark> tags for highlighting
   - Provides defense-in-depth against potential XSS

2. Error State (SearchPage):
   - Add user-facing error message when search fails
   - Display friendly error notification instead of silent failure
   - Improves UX by informing users of temporary issues

Testing:
- All 32 search component tests passing
- TypeScript typecheck passing
- DOMPurify properly sanitizes HTML while preserving highlighting

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

apps/web/src/app/(authenticated)/knowledge/search/page.tsx

@@ -23,6 +23,7 @@ export default function SearchPage(): React.JSX.Element {
   const [results, setResults] = useState<SearchResult[]>([]);
   const [totalResults, setTotalResults] = useState(0);
   const [isLoading, setIsLoading] = useState(false);
+  const [error, setError] = useState<string | null>(null);
   const [selectedTags, setSelectedTags] = useState<string[]>([]);
   const [selectedStatus, setSelectedStatus] = useState<string | undefined>();
   const [availableTags, setAvailableTags] = useState<Tag[]>([]);
@@ -51,6 +52,7 @@ export default function SearchPage(): React.JSX.Element {
       }
 
       setIsLoading(true);
+      setError(null);
       try {
         // Build query params
         const params = new URLSearchParams({ q: query });
@@ -67,6 +69,7 @@ export default function SearchPage(): React.JSX.Element {
         setTotalResults(response.pagination.total);
       } catch (error) {
         console.error("Search failed:", error);
+        setError("Search temporarily unavailable. Please try again in a moment.");
         setResults([]);
         setTotalResults(0);
       } finally {
@@ -99,8 +102,21 @@ export default function SearchPage(): React.JSX.Element {
         </div>
       </div>
 
+      {/* Error state */}
+      {error && (
+        <div className="container mx-auto max-w-6xl mt-6">
+          <div className="bg-yellow-50 border border-yellow-200 rounded-lg p-4 flex items-start gap-3">
+            <span className="text-2xl">⚠️</span>
+            <div className="flex-1">
+              <h3 className="font-semibold text-yellow-900 mb-1">Search Unavailable</h3>
+              <p className="text-yellow-800 text-sm">{error}</p>
+            </div>
+          </div>
+        </div>
+      )}
+
       {/* Results area */}
-      {query && (
+      {query && !error && (
         <div className="container mx-auto max-w-7xl">
           <SearchResults
             results={results}

apps/web/src/components/search/SearchResults.tsx

@@ -1,6 +1,7 @@
 "use client";
 
 import Link from "next/link";
+import DOMPurify from "dompurify";
 import { Card } from "@/components/ui/card";
 import { Badge } from "@/components/ui/badge";
 import { SearchFilters } from "./SearchFilters";
@@ -50,7 +51,12 @@ function SearchResultItem({ result }: { result: SearchResult }): React.JSX.Eleme
           {result.headline && (
             <div
               className="text-sm text-gray-600 line-clamp-2"
-              dangerouslySetInnerHTML={{ __html: result.headline }}
+              dangerouslySetInnerHTML={{
+                __html: DOMPurify.sanitize(result.headline, {
+                  ALLOWED_TAGS: ["mark"],
+                  ALLOWED_ATTR: [],
+                }),
+              }}
             />
           )}