refactor: change model button terminology from Upload to Import (#6800)

## Summary
Changes user-facing text from "Upload" to "Import" for the model import
feature, as "Import" better describes importing models from external
sources like Civitai.

## Changes
- Updated button icon from `upload` to `package-plus`
- Changed all user-facing text in English locale from "Upload" to
"Import"

🤖 Generated with [Claude Code](https://claude.com/claude-code)

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-6800-refactor-change-model-button-terminology-from-Upload-to-Import-2b26d73d365081059ce6e4a548f943ce)
by [Unito](https://www.unito.io)

Co-authored-by: Claude <noreply@anthropic.com>

src/platform/assets/components/AssetBrowserModal.vue

@@ -38,7 +38,7 @@
           :on-click="handleUploadClick"
         >
           <template #icon>
-            <i class="icon-[lucide--upload]" />
+            <i class="icon-[lucide--package-plus]" />
           </template>
         </IconTextButton>
       </div>