⚠️ Rework completely how transitions works

- Use Svelte/Sapper native if and transitions to show either the page content or the loader, then load each page animationIn
- Code is safe on SSR side, using process.browser on this if
- The <main> element is on position absolute to fade nicely the different pages
- Code cleaning

src/style/_base.scss

@@ -141,17 +141,22 @@ button {
         align-items: center;
         justify-content: center;
 
+        @include breakpoint (sm) {
+            flex-direction: row;
+            align-items: baseline;
+        }
+
         em {
             margin: 8px 0 4px;
 
             @include breakpoint (sm) {
                 margin: 2px 16px 0 24px;
             }
-        }
 
-        @include breakpoint (sm) {
-            flex-direction: row;
-            align-items: baseline;
+            span {
+                position: relative;
+                display: inline-block;
+            }
         }
     }
 }