fix: make hover-based buttons accessible on touch devices (#7872)

## Summary
- Define `touch:` Tailwind variant using `@media (hover: none)` to
target touch devices
- Add `touch:opacity-100` to `TreeExplorerTreeNode` for node action
buttons
- Add `useMediaQuery('(hover: none)')` to `MediaAssetCard` for action
overlay visibility

## Problem
On touch devices, sidebar buttons that appear on hover are inaccessible
because:
1. The `touch:` Tailwind variant was used but never defined (classes
silently ignored)
2. `TreeExplorerTreeNode` had no touch support for action buttons
3. `MediaAssetCard` used JS-based `useElementHover` which doesn't work
on touch

## Screenshots (Touch Device Emulation)

### Before (main branch)
- No "Generated"/"Imported" tabs visible in header
- Only duration chips shown on cards, no action buttons (zoom, menu)

![Before - Touch Device](https://i.imgur.com/V0qcr2D.png)

### After (with fix)
- "Generated"/"Imported" tabs visible in header
- Action buttons (zoom, menu) visible on left of cards
- Duration chips moved to right side

![After - Touch Device](https://i.imgur.com/vQ3dUBc.png)

## Test plan
- [ ] On touch device: verify Media Assets sidebar
"Imported"/"Generated" tabs are visible
- [ ] On touch device: verify Node Library filter buttons are visible
- [ ] On touch device: verify tree node action buttons (bookmark, help)
are visible
- [ ] On touch device: verify media asset card zoom/menu buttons are
visible
- [ ] On desktop with mouse: verify hover behavior still works as
expected

packages/design-system/src/css/style.css

@@ -9,6 +9,8 @@
 
 @config '../../tailwind.config.ts';
 
+@custom-variant touch (@media (hover: none));
+
 @theme {
   --text-xxs: 0.625rem;
   --text-xxs--line-height: calc(1 / 0.625);

src/components/common/TreeExplorerTreeNode.vue

@@ -28,7 +28,7 @@
       />
     </div>
     <div
-      class="node-actions motion-safe:opacity-0 motion-safe:group-hover/tree-node:opacity-100"
+      class="node-actions touch:opacity-100 motion-safe:opacity-0 motion-safe:group-hover/tree-node:opacity-100"
     >
       <slot name="actions" :node="props.node" />
     </div>

src/platform/assets/components/MediaAssetCard.vue

@@ -68,9 +68,10 @@
           </IconGroup>
         </template>
 
-        <!-- Output count (top-right) -->
-        <template v-if="showOutputCount" #top-right>
+        <!-- Output count or duration chip (top-right) -->
+        <template v-if="showOutputCount || showTouchDurationChip" #top-right>
           <Button
+            v-if="showOutputCount"
             v-tooltip.top.pt:pointer-events-none="
               $t('mediaAsset.actions.seeMoreOutputs')
             "
@@ -81,6 +82,12 @@
             <i class="icon-[lucide--layers] size-4" />
             <span>{{ outputCount }}</span>
           </Button>
+          <!-- Duration chip on touch devices (far right) -->
+          <SquareChip
+            v-else-if="showTouchDurationChip"
+            variant="gray"
+            :label="formattedDuration"
+          />
         </template>
       </CardTop>
     </template>
@@ -124,7 +131,7 @@
 </template>
 
 <script setup lang="ts">
-import { useElementHover, whenever } from '@vueuse/core'
+import { useElementHover, useMediaQuery, whenever } from '@vueuse/core'
 import { computed, defineAsyncComponent, provide, ref, toRef } from 'vue'
 
 import IconGroup from '@/components/button/IconGroup.vue'
@@ -202,6 +209,7 @@ const showVideoControls = ref(false)
 const imageDimensions = ref<{ width: number; height: number } | undefined>()
 
 const isHovered = useElementHover(cardContainerRef)
+const isTouch = useMediaQuery('(hover: none)')
 
 const actions = useMediaAssetActions()
 
@@ -272,19 +280,28 @@ const durationChipClasses = computed(() => {
   return ''
 })
 
-// Show static chips when NOT hovered and NOT playing (normal state)
+// Show static chips when NOT hovered and NOT playing (normal state on non-touch)
 const showStaticChips = computed(
   () =>
     !loading &&
     !!asset &&
     !isHovered.value &&
     !isVideoPlaying.value &&
+    !isTouch.value &&
     formattedDuration.value
 )
 
-// Show action overlay when hovered OR playing
+// Show duration chip in top-right on touch devices
+const showTouchDurationChip = computed(
+  () => !loading && !!asset && isTouch.value && formattedDuration.value
+)
+
+// Show action overlay when hovered, playing, or on touch device
 const showActionsOverlay = computed(
-  () => !loading && !!asset && (isHovered.value || isVideoPlaying.value)
+  () =>
+    !loading &&
+    !!asset &&
+    (isHovered.value || isVideoPlaying.value || isTouch.value)
 )
 
 const handleZoomClick = () => {