feat(frontend): add simultaneous slide transitions on directory navigation

- Detect navigation direction (forward/backward) via router beforeEach guard
- Store transition direction in Pinia for cross-component access
- Wrap ExplorerView content in a CSS grid transition wrapper so old and new
  views overlap in the same grid cell during animation
- Add slide-forward/slide-backward transition classes with translate3d
  for GPU-accelerated, simultaneous enter/leave without gaps
- Keep fixed-position search loader outside the transition

frontend/src/assets/main.css

@@ -57,6 +57,66 @@
     align-self: stretch;
   }
 }
+/* Directory navigation slide transitions */
+.transition-wrapper {
+  display: grid;
+  grid-template-columns: 1fr;
+  grid-template-rows: 1fr;
+}
+
+.explorer-content {
+  grid-area: 1 / 1;
+}
+
+.slide-forward-enter-active,
+.slide-backward-enter-active {
+  z-index: 2;
+}
+
+.slide-forward-leave-active,
+.slide-backward-leave-active {
+  z-index: 1;
+}
+
+.slide-forward-enter-active,
+.slide-forward-leave-active,
+.slide-backward-enter-active,
+.slide-backward-leave-active {
+  transition: transform 0.22s cubic-bezier(0.32, 0.72, 0, 1);
+}
+
+.slide-forward-enter-from {
+  transform: translate3d(100%, 0, 0);
+}
+
+.slide-forward-enter-to {
+  transform: translate3d(0, 0, 0);
+}
+
+.slide-forward-leave-from {
+  transform: translate3d(0, 0, 0);
+}
+
+.slide-forward-leave-to {
+  transform: translate3d(-100%, 0, 0);
+}
+
+.slide-backward-enter-from {
+  transform: translate3d(-100%, 0, 0);
+}
+
+.slide-backward-enter-to {
+  transform: translate3d(0, 0, 0);
+}
+
+.slide-backward-leave-from {
+  transform: translate3d(0, 0, 0);
+}
+
+.slide-backward-leave-to {
+  transform: translate3d(100%, 0, 0);
+}
+
 @media print {
   :root {
     --primary-color: black;
@@ -206,6 +266,8 @@ main {
   min-height: 0; /* Allow flex child to shrink below content size */
   padding-bottom: 3em; /* convenience space on the bottom */
   overflow-y: scroll;
+  overflow-x: hidden;
+  position: relative;
   text-align: center;
 }