🐛(frontend) fix find & replace crash when editor becomes read-only

BlockNoteReader (read-only docs) doesn't register the find & replace
tiptap extension. If the panel is left open while the active editor
switches to it -- e.g. a collaborative doc turning read-only after a
WebSocket disconnect -- calling into its commands threw
"commands.clearSearch is not a function".

useFindReplace now exposes whether the current editor actually
supports find & replace, and FindReplace closes the panel and renders
nothing when it doesn't, instead of crashing.

src/frontend/apps/impress/src/features/docs/doc-find-replace/components/FindReplace.tsx

@@ -36,6 +36,7 @@ export const FindReplace = () => {
   const findInputRef = useRef<HTMLInputElement>(null);
 
   const {
+    isSupported,
     query,
     setQuery,
     replacement,
@@ -70,6 +71,16 @@ export const FindReplace = () => {
     findInputRef.current?.select();
   }, [editor?._tiptapEditor, setQuery, openCount]);
 
+  // The editor can lose find & replace support while the panel is open, e.g.
+  // when a collaborative doc briefly turns read-only after a WebSocket
+  // disconnect. Close the panel so the floating bar falls back to its normal
+  // controls instead of rendering nothing.
+  useEffect(() => {
+    if (!isSupported) {
+      close();
+    }
+  }, [isSupported, close]);
+
   const handleClose = () => {
     close();
     restoreFocus();
@@ -151,6 +162,10 @@ export const FindReplace = () => {
     handleReplaceCurrent();
   };
 
+  if (!isSupported) {
+    return null;
+  }
+
   return (
     <Card
       role="search"

src/frontend/apps/impress/src/features/docs/doc-find-replace/hooks/useFindReplace.ts

@@ -9,7 +9,9 @@ export const useFindReplace = (editor: DocsBlockNoteEditor | undefined) => {
   const [matchCount, setMatchCount] = useState(0);
   const [activeIndex, setActiveIndex] = useState(-1);
 
-  const tiptapEditor = editor?._tiptapEditor;
+  const isSupported =
+    typeof editor?._tiptapEditor.commands.clearSearch === 'function';
+  const tiptapEditor = isSupported ? editor?._tiptapEditor : undefined;
 
   const syncFromStorage = useCallback(() => {
     if (!tiptapEditor) {
@@ -111,6 +113,7 @@ export const useFindReplace = (editor: DocsBlockNoteEditor | undefined) => {
   }, [tiptapEditor]);
 
   return {
+    isSupported,
     query,
     setQuery,
     replacement,