feat(auth): rebuild the sign-in surfaces on the shared sign-card contract

The login/register page and the auth modal had drifted into a parallel
system: floating-input fields, a sign-page-card shell and a bespoke
jio-auth-modal wrapper that none of the theme's own sign CSS targeted.
The reference card rules (.sign, .sign-img, .sign-content, .line-form-input,
.scale-placeholder, .social_loginbar, .title-h-left, #u_sign .modal-dialog,
.qrcode-signin-container) have been in main.css all along, so the fix is to
render the reference DOM instead of re-styling a second one.

auth-content.php now emits one .sign.jio-panel.blur-bg.relative card with a
.tab-content of #tab-sign-in / #tab-sign-up / #tab-resetpassword /
#tab-qrcode-signin / #tab-bind panes. Each pane opens with a
.title-h-left.fa-2x heading plus a .muted-color.px12 link, fields are
.relative.line-form.mb10 > .line-form-input + .scale-placeholder +
.line-form-line, the verification trigger is the .yztx.abs-right captchsubmit
button, and submits are .but.radius.jb-blue|jb-green.padding-lg.btn-block.
Social entries come from the shared render_social_buttons() separator +
.social_loginbar, and the OAuth bind pane uses the oauth-data-box avatar card.

The modal shell is #u_sign.modal > .modal-dialog > .sign-content >
.sign-img.absolute.hide-sm + .sign, and the page shell is the reference
.container.sign-page > .row.sign-row.gutters-5 > .col-md-6(.col-md-offset-6/
.col-md-offset-3).sign-col with a .fixed.lazyload background and a blurred
fixed footer. auth.js drives .in instead of .is-open, toggles the new
password/signin-mode/invite-gate controls and no longer touches the retired
markup. The page also drops two footer-hook removals that targeted functions
which no longer exist, and opts out of the fixed-nav offset.

The sign page's custom theme setting now actually applies: Router answers
jio_forced_theme_mode, jioData.themeForced wins over the stored cookie in
theme.js, theme-toggle positions are suppressed, and the page marks html/body
dark or white accordingly.

sign.css shrinks from ~114 lines of bespoke component styling to a thin
positioning layer, and the retired classes are gone rather than kept.

Verification: php -l on every changed PHP file, node --check on both JS files,
node --test across widget-system, navigation and front-submit (873 passing),
and tools/audit-psr4-paths.php / audit-admin-contracts.php both at 0 errors.
assets/js/runtime/main.min.js was rebuilt because theme.js gained the forced
theme branch.

templates/parts/modal-auth.php

@@ -17,7 +17,6 @@ $image_id = $images ? (int) $images[array_rand($images)] : 0;
 $image_html = $image_id > 0 ? wp_get_attachment_image($image_id, 'full', false, [
 	'class'    => 'fit-cover radius8',
 	'alt'      => get_bloginfo('name'),
-	'loading'  => 'lazy',
 	'decoding' => 'async',
 ]) : '';
 $auto = class_exists('\Jio\Settings\Settings') ? \Jio\Settings\Settings::sign_modal_auto() : ['enabled' => false, 'waiting' => 5, 'expires' => 1];
@@ -30,13 +29,11 @@ $jio_auth_context = [
 	'redirect_to' => home_url('/'),
 ];
 ?>
-<div id="u_sign" class="jio-auth-modal<?php echo $image_html !== '' ? ' has-sign-img' : ''; ?>" aria-hidden="true" role="dialog" aria-modal="true" aria-labelledby="jio-auth-modal-title" tabindex="-1" data-auth-modal data-auto="<?php echo !empty($auto['enabled']) ? '1' : '0'; ?>" data-auto-wait="<?php echo esc_attr((string) $auto['waiting']); ?>" data-auto-expires="<?php echo esc_attr((string) $auto['expires']); ?>">
-		<h2 id="jio-auth-modal-title" class="screen-reader-text"><?php esc_html_e('账户登录与注册', 'jio'); ?></h2>
-	<div class="auth-modal-backdrop" data-auth-close></div>
-	<div class="modal-dialog">
-		<div class="modal-content">
-			<button type="button" class="auth-modal-close" data-auth-close aria-label="<?php esc_attr_e('关闭', 'jio'); ?>"><i class="fa fas fa-times"></i></button>
-			<?php if ($image_html !== '') : ?><div class="sign-img"><?php echo $image_html; // phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped ?></div><?php endif; ?>
+<div id="u_sign" class="modal jio-auth-modal<?php echo $image_html !== '' ? ' has-sign-img' : ''; ?>" aria-hidden="true" role="dialog" aria-modal="true" aria-labelledby="jio-auth-modal-title" tabindex="-1" data-auth-modal data-auto="<?php echo !empty($auto['enabled']) ? '1' : '0'; ?>" data-auto-wait="<?php echo esc_attr((string) $auto['waiting']); ?>" data-auto-expires="<?php echo esc_attr((string) $auto['expires']); ?>">
+	<h2 id="jio-auth-modal-title" class="screen-reader-text"><?php esc_html_e('账户登录与注册', 'jio'); ?></h2>
+	<div class="modal-dialog" role="document">
+		<div class="sign-content">
+			<?php if ($image_html !== '') : ?><div class="sign-img absolute hide-sm"><?php echo $image_html; // phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped ?></div><?php endif; ?>
 			<?php require get_template_directory() . '/templates/parts/auth-content.php'; ?>
 		</div>
 	</div>