Add hover cards in web UI (#30754)

Co-authored-by: Renaud Chaput <renchap@gmail.com>

app/javascript/mastodon/components/follow_button.tsx

@@ -0,0 +1,93 @@
+import { useCallback, useEffect } from 'react';
+
+import { useIntl, defineMessages } from 'react-intl';
+
+import {
+  fetchRelationships,
+  followAccount,
+  unfollowAccount,
+} from 'mastodon/actions/accounts';
+import { Button } from 'mastodon/components/button';
+import { LoadingIndicator } from 'mastodon/components/loading_indicator';
+import { me } from 'mastodon/initial_state';
+import { useAppDispatch, useAppSelector } from 'mastodon/store';
+
+const messages = defineMessages({
+  unfollow: { id: 'account.unfollow', defaultMessage: 'Unfollow' },
+  follow: { id: 'account.follow', defaultMessage: 'Follow' },
+  followBack: { id: 'account.follow_back', defaultMessage: 'Follow back' },
+  mutual: { id: 'account.mutual', defaultMessage: 'Mutual' },
+  cancel_follow_request: {
+    id: 'account.cancel_follow_request',
+    defaultMessage: 'Withdraw follow request',
+  },
+  edit_profile: { id: 'account.edit_profile', defaultMessage: 'Edit profile' },
+});
+
+export const FollowButton: React.FC<{
+  accountId: string;
+}> = ({ accountId }) => {
+  const intl = useIntl();
+  const dispatch = useAppDispatch();
+  const relationship = useAppSelector((state) =>
+    state.relationships.get(accountId),
+  );
+  const following = relationship?.following || relationship?.requested;
+
+  useEffect(() => {
+    dispatch(fetchRelationships([accountId]));
+  }, [dispatch, accountId]);
+
+  const handleClick = useCallback(() => {
+    if (!relationship) return;
+    if (accountId === me) {
+      return;
+    } else if (relationship.following || relationship.requested) {
+      dispatch(unfollowAccount(accountId));
+    } else {
+      dispatch(followAccount(accountId));
+    }
+  }, [dispatch, accountId, relationship]);
+
+  let label;
+
+  if (accountId === me) {
+    label = intl.formatMessage(messages.edit_profile);
+  } else if (!relationship) {
+    label = <LoadingIndicator />;
+  } else if (relationship.requested) {
+    label = intl.formatMessage(messages.cancel_follow_request);
+  } else if (relationship.following && relationship.followed_by) {
+    label = intl.formatMessage(messages.mutual);
+  } else if (!relationship.following && relationship.followed_by) {
+    label = intl.formatMessage(messages.followBack);
+  } else if (relationship.following) {
+    label = intl.formatMessage(messages.unfollow);
+  } else {
+    label = intl.formatMessage(messages.follow);
+  }
+
+  if (accountId === me) {
+    return (
+      <a
+        href='/settings/profile'
+        target='_blank'
+        rel='noreferrer noopener'
+        className='button button-secondary'
+      >
+        {label}
+      </a>
+    );
+  }
+
+  return (
+    <Button
+      onClick={handleClick}
+      disabled={relationship?.blocked_by || relationship?.blocking}
+      secondary={following}
+      className={following ? 'button--destructive' : undefined}
+    >
+      {label}
+    </Button>
+  );
+};