fix(auth): align the social-login surfaces with the reference theme

The "other login methods" block drifted from zibll on three axes. Gitee was
mapped to fa-code-branch because FA5 Free ships no Gitee brand glyph, so the
button showed a generic branch icon; Google showed the monochrome FA glyph
instead of the reference's multi-colour mark. The modal also printed an
implicit agreement line under the social buttons, which the reference guest
card does not have. And the top-nav user popover built its own
.social-login-divider / .oauth-row / .oauth-btn markup and then wrapped that
in a second separator and .social_loginbar, so the block rendered nested and
unlike the reference.

aksrt_brand_icon() now routes zibsvg-/customsvg- values through the shared
icon renderer, and the brand map resolves gitee to zibsvg-gitee and google to
zibsvg-google-color. Both sprite keys already exist in the runtime symbol
sheet, which the loader pulls in on every frontend page. Because
get_auth_providers_data() and binding_row() both go through
aksrt_brand_icon(), one map change fixes the icon on the modal, the user card,
the top-nav popover, the user-center binding list and the profile social
links.

render_social_buttons() is rewritten to the reference shape: an optional
.social-separator, a .social_loginbar, and one .social-login-item <type>
toggle-radius anchor per enabled provider with rel="nofollow" and a
"<label>登录" title. oauth_button_lg switches to the button-lg variant with a
label, and the WeChat entry carries qrcode-signin so auth.js opens the QR
panel instead of navigating, matching the reference. The navbar and the link
apply modal now append the block verbatim instead of re-wrapping it, and the
user-card loop adopts the same item attributes. The retired
.social-login-divider / .oauth-row / .oauth-btn rules are dropped from
base-1.css, and the binding-list icon rule gains an svg selector so the
sprite icons keep the 22px the FA glyphs had.

Verification: node --test across widget-system, navigation, admin,
asset-ledger, dev, front-submit and visual (613 tests), plus php -l on every
changed PHP file. Extended the auth source contract with the agreement-line,
brand-icon and popover-structure cases, and pinned the user-card guest
markup.

templates/parts/auth-content.php

@@ -147,6 +147,5 @@ $oauth_bind_pass = $oauth_intent === 'bind'
 		<?php foreach ($oauth_providers as $provider) : ?><?php if (($provider['name'] ?? '') === 'weixingzh') continue; ?><?php $provider_label = (string) ($provider['label'] ?? $provider['name'] ?? ''); ?><a class="social-login-item <?php echo esc_attr((string) ($provider['name'] ?? '')); ?>" href="<?php echo esc_url((string) ($provider['url'] ?? '#')); ?>" data-provider="<?php echo esc_attr((string) ($provider['name'] ?? '')); ?>" aria-label="<?php echo esc_attr(sprintf(__('%s登录', 'aksrt'), $provider_label)); ?>" title="<?php echo esc_attr($provider_label); ?>"><?php echo $provider['icon'] ?? ''; // phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped ?><?php if ($oauth_large) : ?><span><?php printf(esc_html__('%s登录', 'aksrt'), esc_html($provider_label)); ?></span><?php endif; ?></a><?php endforeach; ?>
 		<?php if ($wechat_qr) : ?><button type="button" class="social-login-item weixingzh" data-auth-tab="oauth" aria-label="<?php esc_attr_e('微信扫码登录', 'aksrt'); ?>" title="<?php esc_attr_e('微信扫码登录', 'aksrt'); ?>"><i class="fa fab fa-weixin" aria-hidden="true"></i><?php if ($oauth_large) : ?><span><?php esc_html_e('微信登录', 'aksrt'); ?></span><?php endif; ?></button><?php endif; ?>
 	</div></div>
-	<?php if (!empty($agreement['enabled'])) : ?><div class="sign-agreement sign-agreement-implicit"><?php esc_html_e('使用社交账号登录即表示', 'aksrt'); ?><?php echo $agreement['label_html']; // phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped ?></div><?php endif; ?>
 	<?php endif; ?>
 </div>