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);