feat(shell): polish header/hero overlay and footer zibll classes

Make show-slide header overlay the hero, dual-class nav/footer shells
toward zibll selectors, and add a small footer layout bridge for logo,
contact, and float buttons.

assets/css/frontend/footer.css

@@ -2,3 +2,4 @@
 @import url("footer/part-2.css");
 @import url("footer/part-3.css");
 @import url("footer/part-4.css");
+@import url("footer/zibll-shell.css");