fix: replace text-white with theme-aware color tokens (#7908)

## Summary
- Replace hardcoded `text-white` class with theme-aware alternatives to
fix invisible text on light themes
- Update Load3D control backgrounds to use semantic tokens
- Update dropdown menus to use `bg-interface-menu-surface`
- Update overlay backgrounds to use `bg-backdrop` with opacity

## Changes
| Component | Old | New |
|-----------|-----|-----|
| Text on primary bg | `text-white` | `text-base-foreground` |
| Dropdown menus | `bg-black/50` | `bg-interface-menu-surface` |
| Control panels | `bg-smoke-700/30` | `bg-backdrop/30` |
| Loading overlays | `bg-black bg-opacity-50` | `bg-backdrop/50` |
| Selected states | `bg-smoke-600` | `bg-button-active-surface` |

## Files Modified (14)
- `src/components/TopMenuSection.vue`
- `src/components/input/MultiSelect.vue`
- `src/components/load3d/*.vue` (12 files)
- `src/renderer/extensions/vueNodes/VideoPreview.vue`

## Test plan
- [ ] Verify text visibility in light theme
- [ ] Verify text visibility in dark theme
- [ ] Test Load3D viewer controls functionality
- [ ] Test MultiSelect dropdown checkbox visibility

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-7908-fix-replace-text-white-with-theme-aware-color-tokens-2e26d73d36508107bb01d1d6e3b74f6a)
by [Unito](https://www.unito.io)

src/components/load3d/controls/PopupSlider.vue

@@ -8,11 +8,11 @@
       :aria-label="tooltipText"
       @click="toggleSlider"
     >
-      <i :class="['pi', icon, 'text-lg text-white']" />
+      <i :class="['pi', icon, 'text-lg text-base-foreground']" />
     </Button>
     <div
       v-show="showSlider"
-      class="absolute top-0 left-12 rounded-lg bg-black/50 p-4 shadow-lg w-[150px]"
+      class="absolute top-0 left-12 rounded-lg bg-interface-menu-surface p-4 shadow-lg w-[150px]"
     >
       <Slider
         v-model="value"