Change floating action button to be a button in header in web UI (#19422)

- Fix theme color
- Fix elephant being too big on error page on small screens
- Remove "Follows and Followers" link from navigation panel

app/javascript/styles/mastodon/components.scss

@@ -2565,30 +2565,6 @@ $ui-header-height: 55px;
   }
 }
 
-.floating-action-button {
-  position: fixed;
-  display: flex;
-  justify-content: center;
-  align-items: center;
-  width: 3.9375rem;
-  height: 3.9375rem;
-  bottom: 1.3125rem;
-  right: 1.3125rem;
-  background: darken($ui-highlight-color, 2%);
-  color: $white;
-  border-radius: 50%;
-  font-size: 21px;
-  line-height: 21px;
-  text-decoration: none;
-  box-shadow: 2px 3px 9px rgba($base-shadow-color, 0.4);
-
-  &:hover,
-  &:focus,
-  &:active {
-    background: $ui-highlight-color;
-  }
-}
-
 @media screen and (min-width: $no-gap-breakpoint) {
   .tabs-bar {
     width: 100%;
@@ -2603,7 +2579,6 @@ $ui-header-height: 55px;
     margin-bottom: 10px;
   }
 
-  .floating-action-button,
   .tabs-bar__link.optional {
     display: none;
   }
@@ -2620,10 +2595,6 @@ $ui-header-height: 55px;
 @media screen and (max-width: $no-gap-breakpoint - 1px) {
   $sidebar-width: 285px;
 
-  .with-fab .scrollable .item-list:last-child {
-    padding-bottom: 5.25rem;
-  }
-
   .columns-area__panels__main {
     width: calc(100% - $sidebar-width);
   }
@@ -4350,6 +4321,7 @@ a.status-card.compact:hover {
   cursor: default;
 
   &__image {
+    width: 70%;
     max-width: 350px;
     margin-top: -50px;
   }
@@ -7270,6 +7242,7 @@ noscript {
     &__buttons {
       display: flex;
       align-items: center;
+      gap: 8px;
       padding-top: 55px;
       overflow: hidden;
 
@@ -7279,14 +7252,6 @@ noscript {
         box-sizing: content-box;
         padding: 2px;
       }
-
-      & > .icon-button {
-        margin-right: 8px;
-      }
-
-      .button {
-        margin: 0 8px;
-      }
     }
 
     &__name {