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/ExportControls.vue

@@ -12,18 +12,18 @@
         :aria-label="$t('load3d.exportModel')"
         @click="toggleExportFormats"
       >
-        <i class="pi pi-download text-lg text-white" />
+        <i class="pi pi-download text-lg text-base-foreground" />
       </Button>
       <div
         v-show="showExportFormats"
-        class="absolute top-0 left-12 rounded-lg bg-black/50 shadow-lg"
+        class="absolute top-0 left-12 rounded-lg bg-interface-menu-surface shadow-lg"
       >
         <div class="flex flex-col">
           <Button
             v-for="format in exportFormats"
             :key="format.value"
             variant="textonly"
-            class="text-white"
+            class="text-base-foreground"
             @click="exportModel(format.value)"
           >
             {{ format.label }}