fix(10-13): sidebar shimmer rows and search-at-root visibility

- TreeItem.vue: replace 'Loading…' text with 3 animate-pulse shimmer rows
- StorageBrowser.vue: showSearch now true for mode=local OR mode=cloud (Gap 1 + Gap 2)

frontend/src/components/storage/StorageBrowser.vue

@@ -284,7 +284,7 @@ const emit = defineEmits([
   'file-delete',
 ])
 
-const showSearch = computed(() => props.mode === 'local' && props.breadcrumb.length > 0)
+const showSearch = computed(() => props.mode === 'local' || props.mode === 'cloud')
 
 function topicColor(name) {
   return props.topicColorFn(name)

frontend/src/components/ui/TreeItem.vue

@@ -46,10 +46,13 @@
     <template v-if="expanded">
       <div
         v-if="loading"
-        class="text-xs text-gray-400 py-1"
+        class="py-1 space-y-1"
         :style="{ paddingLeft: `${(depth + 1) * 12 + 8}px` }"
       >
-        Loading…
+        <div v-for="n in 3" :key="`sk-t-${n}`" class="flex items-center gap-2 py-1">
+          <div class="w-4 h-4 bg-gray-100 rounded animate-pulse shrink-0"></div>
+          <div class="h-3 bg-gray-100 rounded animate-pulse" :style="{ width: (50 + n * 15) + 'px' }"></div>
+        </div>
       </div>
       <div
         v-else-if="loadError"