fix(footer): stop day/night brand logos both showing

Size rules were forcing display:inline-block on both footer logos with
higher specificity than the night-hide rule. Decouple display toggle,
raise selector weight, align dark hooks with header/theme.js, and drop
unused part-1 light/dark logo-img rules.

assets/css/frontend/footer/part-1.css

@@ -177,7 +177,7 @@
 
 /* §2.3 三列结构化字段 */
 
-/* 品牌 Logo */
+/* 品牌 Logo — day/night visibility lives in zibll-shell.css */
 .site-footer__logo {
     margin-bottom: 16px;
 }
@@ -185,15 +185,6 @@
     max-height: 48px;
     width: auto;
 }
-.site-footer__logo-img--dark {
-    display: none;
-}
-[data-theme="dark"] .site-footer__logo-img--light {
-    display: none;
-}
-[data-theme="dark"] .site-footer__logo-img--dark {
-    display: inline-block;
-}
 
 /* 标准联系项 */
 .site-footer__contact-std {

assets/css/frontend/footer/zibll-shell.css

@@ -53,21 +53,27 @@
   height: 40px;
   width: auto;
   max-width: 180px;
-  display: inline-block;
   vertical-align: middle;
 }
 
-.footer .site-footer__logo--night {
+/* Light/dark footer logo toggle — keep display out of size rules so hide wins */
+.footer a.footer-logo .site-footer__logo--day {
+  display: inline-block;
+}
+
+.footer a.footer-logo .site-footer__logo--night {
   display: none;
 }
 
-html.dark .footer .site-footer__logo--day,
-body.dark-theme .footer .site-footer__logo--day {
+html.dark .footer a.footer-logo .site-footer__logo--day,
+[data-theme="dark"] .footer a.footer-logo .site-footer__logo--day,
+body.dark-theme .footer a.footer-logo .site-footer__logo--day {
   display: none;
 }
 
-html.dark .footer .site-footer__logo--night,
-body.dark-theme .footer .site-footer__logo--night {
+html.dark .footer a.footer-logo .site-footer__logo--night,
+[data-theme="dark"] .footer a.footer-logo .site-footer__logo--night,
+body.dark-theme .footer a.footer-logo .site-footer__logo--night {
   display: inline-block;
 }