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.

inc/helpers/header/aksrt-header-navbar.php

@@ -286,11 +286,7 @@ if (!function_exists('aksrt_header_user_box_html')) :
 
 			$con = '<div class="text-center">' . $href . '</div>';
 			if (class_exists('\AKSRT\Auth\OAuth') && method_exists(\AKSRT\Auth\OAuth::get_instance(), 'render_social_buttons')) {
-				$oauth_html = (string) \AKSRT\Auth\OAuth::get_instance()->render_social_buttons(__('快速登录', 'aksrt'));
-				if ($oauth_html !== '') {
-					$con .= '<p class="social-separator separator muted-3-color em09 mt10">' . esc_html__('快速登录', 'aksrt') . '</p>';
-					$con .= '<div class="social_loginbar">' . $oauth_html . '</div>';
-				}
+				$con .= (string) \AKSRT\Auth\OAuth::get_instance()->render_social_buttons(__('快速登录', 'aksrt'));
 			}
 		}