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;
 }
 

frontend/src/router/index.ts

@@ -1,6 +1,12 @@
+import { useMainStore } from '@/stores/main'
 import ExplorerView from '@/views/ExplorerView.vue'
 import { createRouter, createWebHashHistory } from 'vue-router'
 
+function getPathDepth(path: string): number {
+  const pathPart = decodeURIComponent(path).split('//')[0] ?? ''
+  return pathPart.split('/').filter(Boolean).length
+}
+
 const router = createRouter({
   history: createWebHashHistory(import.meta.env.BASE_URL),
   routes: [
@@ -12,4 +18,17 @@ const router = createRouter({
   ]
 })
 
+router.beforeEach((to, from) => {
+  const store = useMainStore()
+  const toDepth = getPathDepth(to.path)
+  const fromDepth = getPathDepth(from.path)
+  if (toDepth > fromDepth) {
+    store.transitionDirection = 'forward'
+  } else if (toDepth < fromDepth) {
+    store.transitionDirection = 'backward'
+  } else {
+    store.transitionDirection = 'none'
+  }
+})
+
 export default router

frontend/src/stores/main.ts

@@ -98,6 +98,7 @@ export const useMainStore = defineStore('main', {
       privileged: false as boolean,
       isLoggedIn: false as boolean
     },
+    transitionDirection: 'none' as 'forward' | 'backward' | 'none',
     space: {
       disk: 0,
       free: 0,

frontend/src/views/ExplorerView.vue

@@ -1,24 +1,32 @@
 <template>
+  <div class="transition-wrapper">
+    <Transition
+      :name="transitionName"
+      @after-enter="store.transitionDirection = 'none'"
+    >
+      <div :key="folderPath" class="explorer-content">
         <Gallery
           v-if="store.prefs.gallery"
           ref="fileExplorer"
-    :key="`gallery-${folderPath}`"
           :path="props.path"
           :documents="documents"
         />
         <FileExplorer
           v-else
           ref="fileExplorer"
-    :key="`explorer-${folderPath}`"
           :path="props.path"
           :documents="documents"
         />
+        <EmptyFolder :documents="documents" :path="props.path" />
+      </div>
+    </Transition>
+  </div>
   <div v-if="store.searchLoading" class="search-loading">Searching...</div>
-  <EmptyFolder :documents=documents :path=props.path />
 </template>
 
 <script setup lang="ts">
 import FileExplorer from '@/components/FileExplorer.vue'
+import Gallery from '@/components/Gallery.vue'
 import { getDocuments } from '@/stores/documentStore'
 import { useMainStore } from '@/stores/main'
 import { collator } from '@/utils'
@@ -35,6 +43,12 @@ const props = defineProps<{
 // Folder path for component keys - only recreate component when folder changes, not search
 const folderPath = computed(() => props.path.join('/'))
 
+const transitionName = computed(() => {
+  if (store.transitionDirection === 'forward') return 'slide-forward'
+  if (store.transitionDirection === 'backward') return 'slide-backward'
+  return ''
+})
+
 // Handle route-based search changes (back/forward navigation, direct URL)
 // Skip if store.query already matches (means we triggered this via typing)
 watch(