fix(auth): close the modal regressions and keep the header component off /sign/

Three defects slipped into the sign-card rebuild:

The hidden-glyph rule was dropped. `.fa` sets display:inline-block, which
outranks the UA [hidden] rule, so every hidden glyph stayed visible — most
visibly a spinner rendered beside the login button at rest. `.sign [hidden]`
restores the override.

Closing account login produced an empty shell. The whole sign-in pane was
gated on `!$close_signin`, and the social block carried the same gate, so a
site with account login off but registration on still opened on an empty
panel with no social entry. The pane is now rendered in three shapes: open
registration falls through to the signup tab, a QR entry falls through to
the QR tab, social-only renders a notice plus the social bar, and when
nothing is available it states that login is closed.

The modal close button sat in normal flow and pushed the card content down;
`#u_sign .sign > .close` pins it back to the top-right corner.

The /sign/ route is `index.php?jio_sign=1`, so WordPress reports it as home.
The header multifunction component resolves page type by query flags and the
home branch won, so the component appeared on the auth page whenever "首页"
was in its scope — even though the sign page template itself was already
excluded. `jio_header_slide_current_page_type()` now bails out on `jio_sign`
before any of the is_* checks.

The breakpoint overrides that were fighting the shared sheet between 641px
and 991px are narrowed to the one offset that actually needed adjusting.

Verification: the modal was rendered through a PHP stub, served with the real
theme stylesheets, and driven by the real auth.js in a headless browser —
tab switching, signin-mode switching, password reveal, close button and
geometry at 1440/1100/900/760/500/390px. The header harness gained a
`jio_sign` case asserting no markup, and 860 widget/navigation tests pass.

templates/parts/auth-content.php

@@ -63,14 +63,16 @@ $oauth_bind_pass = $oauth_intent === 'bind'
 	: '';
 
 $initial_tab = $auth_tab;
-if ($initial_tab === 'signin' && $close_signin && $registration_open) {
-	$initial_tab = 'signup';
+if ($initial_tab === 'signin' && $close_signin) {
+	// Account login is off: land on registration when it is open, otherwise on
+	// the social/QR pane so the modal never opens on an empty sign-in panel.
+	$initial_tab = $registration_open ? 'signup' : ($wechat_qr ? 'oauth' : 'signin');
 }
 if ($initial_tab === 'signup' && !$registration_open) {
 	$initial_tab = 'signin';
 }
 if ($initial_tab === 'oauth' && !$wechat_qr) {
-	$initial_tab = 'signin';
+	$initial_tab = $close_signin && $registration_open ? 'signup' : 'signin';
 }
 if ($initial_tab === 'bind' && !$is_oauth_bind) {
 	$initial_tab = 'signin';
@@ -81,7 +83,7 @@ $show_pas = !$passwordless || $passwordless_mode !== 'only_nopas';
 $nopas_active = $show_nopas && $passwordless_mode !== 'pas';
 $signin_user_hint = ($passwordless || $signin_phone) ? __('用户名/手机号/邮箱', 'jio') : __('用户名或邮箱', 'jio');
 $social_html = '';
-if (!$is_oauth_bind && !$close_signin && !empty($oauth_providers) && class_exists('\Jio\Auth\OAuth')) {
+if (!$is_oauth_bind && !empty($oauth_providers) && class_exists('\Jio\Auth\OAuth')) {
 	$social_html = \Jio\Auth\OAuth::get_instance()->render_social_buttons(__('社交账号登录', 'jio'));
 }
 $ref_tooltip = $ref_hint !== '' ? wp_strip_all_tags($ref_hint) : '';
@@ -96,7 +98,24 @@ $invite_tooltip = ($invite_mode === 'optional' && $invite_hint !== '') ? wp_stri
 	<?php endif; ?>
 
 	<div class="tab-content">
-		<?php if (!$close_signin) : ?>
+		<?php if ($close_signin && !$registration_open && !$social_html) : ?>
+		<div class="tab-pane fade active in" id="<?php echo esc_attr($unique); ?>-tab-sign-in" role="tabpanel" data-auth-panel="signin">
+			<div class="box-body">
+				<div class="title-h-left fa-2x"><?php esc_html_e('登录', 'jio'); ?></div>
+			</div>
+			<div class="box-body"><div class="c-red px12"><i class="fa fas fa-info-circle mr6" aria-hidden="true"></i><?php esc_html_e('当前已关闭登录，请稍后再试。', 'jio'); ?></div></div>
+		</div>
+		<?php elseif ($close_signin) : ?>
+		<div class="tab-pane fade<?php echo $initial_tab === 'signin' ? ' active in' : ''; ?>" id="<?php echo esc_attr($unique); ?>-tab-sign-in" role="tabpanel" data-auth-panel="signin" <?php echo $initial_tab !== 'signin' ? 'hidden' : ''; ?>>
+			<div class="box-body">
+				<div class="title-h-left fa-2x"><?php esc_html_e('登录', 'jio'); ?></div>
+				<?php if ($registration_open) : ?><a class="muted-color px12" href="#<?php echo esc_attr($unique); ?>-tab-sign-up" data-auth-tab="signup"><?php esc_html_e('没有账号？立即注册', 'jio'); ?><i class="em12 ml3 fa fas fa-angle-right" aria-hidden="true"></i></a><?php endif; ?>
+			</div>
+			<div class="box-body"><div class="muted-2-color px12"><i class="fa fas fa-info-circle mr6" aria-hidden="true"></i><?php esc_html_e('当前已关闭账号密码登录，请使用第三方登录。', 'jio'); ?></div></div>
+			<?php echo $social_html; // phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped ?>
+			<?php if ($social_html !== '' && !empty($agreement['enabled'])) : ?><div class="muted-color mt10 text-center px12 opacity8"><?php esc_html_e('使用社交账号登录即表示同意', 'jio'); ?><?php echo $agreement['links_html']; // phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped ?></div><?php endif; ?>
+		</div>
+		<?php else : ?>
 		<div class="tab-pane fade<?php echo $initial_tab === 'signin' ? ' active in' : ''; ?>" id="<?php echo esc_attr($unique); ?>-tab-sign-in" role="tabpanel" data-auth-panel="signin" <?php echo $initial_tab !== 'signin' ? 'hidden' : ''; ?>>
 			<div class="box-body">
 				<div class="title-h-left fa-2x"><?php esc_html_e('登录', 'jio'); ?></div>