Simplify i18n: remove old react-intl artifacts, rename formatMessage to t, replace FormattedMessage with t.rich().

- Rewrite messages.ts to plain string key maps (remove MessageDescriptor)
- Rewrite useMessages hook to expose t from useTranslations() directly
- Rename formatMessage → t across 193 consumer files
- Replace custom FormattedMessage component with next-intl t.rich()
- Update 52 language files to use rich text tags (<b>, <a>)
- Remove all direct imports from @/components/messages in favor of useMessages()

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

src/app/(main)/boards/BoardProvider.tsx

@@ -40,7 +40,7 @@ export function BoardProvider({
   const { post, useMutation } = useApi();
   const { touch } = useModified();
   const { toast } = useToast();
-  const { formatMessage, labels, messages } = useMessages();
+  const { t, labels, messages } = useMessages();
   const { router, renderUrl } = useNavigation();
 
   const [board, setBoard] = useState<Partial<Board>>(data ?? createDefaultBoard());
@@ -70,7 +70,7 @@ export function BoardProvider({
   }, []);
 
   const saveBoard = useCallback(async () => {
-    const defaultName = formatMessage(labels.untitled);
+    const defaultName = t(labels.untitled);
 
     // Get current layout sizes from BoardBody if registered
     const layoutData = layoutGetterRef.current?.();
@@ -82,7 +82,7 @@ export function BoardProvider({
       parameters,
     });
 
-    toast(formatMessage(messages.saved));
+    toast(t(messages.saved));
     touch('boards');
 
     if (board.id) {
@@ -92,17 +92,7 @@ export function BoardProvider({
     }
 
     return result;
-  }, [
-    board,
-    mutateAsync,
-    toast,
-    formatMessage,
-    labels.untitled,
-    messages.saved,
-    touch,
-    router,
-    renderUrl,
-  ]);
+  }, [board, mutateAsync, toast, t, labels.untitled, messages.saved, touch, router, renderUrl]);
 
   if (boardId && isFetching && isLoading) {
     return <Loading placement="absolute" />;