[feat] Add feature-flagged upload button to asset browser (#6665)

## Summary
Adds an upload button to the asset browser modal, controlled by the
`model_upload_button_enabled` backend feature flag.

## Changes
- **What**: Added upload button with PrimeVue primary styling to asset
browser header
- **Feature Flag**: Button only appears when backend returns
`model_upload_button_enabled: true`
- **Localization**: Added `assetBrowser.uploadModel` translation key
- **Click Handler**: Currently logs to console (implementation pending)

## Review Focus
- Feature flag integration using `useFeatureFlags` composable
- Button styling matches PrimeVue primary color scheme
- Proper placement in header with flexbox layout

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-6665-feat-Add-feature-flagged-upload-button-to-asset-browser-2a96d73d365081c7a05bdc33bed7f7fd)
by [Unito](https://www.unito.io)

---------

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

src/locales/en/main.json

@@ -1984,6 +1984,7 @@
     "failedToCreateNode": "Failed to create node. Please try again or check console for details.",
     "noModelsInFolder": "No {type} available in this folder",
     "searchAssetsPlaceholder": "Type to search...",
+    "uploadModel": "Upload model",
     "allModels": "All Models",
     "allCategory": "All {category}",
     "unknown": "Unknown",