[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/platform/assets/components/AssetBrowserModal.vue

@@ -21,13 +21,27 @@
     </template>
 
     <template #header>
-      <SearchBox
-        v-model="searchQuery"
-        :autofocus="true"
-        size="lg"
-        :placeholder="$t('assetBrowser.searchAssetsPlaceholder')"
-        class="max-w-96"
-      />
+      <div class="flex w-full items-center justify-between gap-2">
+        <SearchBox
+          v-model="searchQuery"
+          :autofocus="true"
+          size="lg"
+          :placeholder="$t('assetBrowser.searchAssetsPlaceholder')"
+          class="max-w-96"
+        />
+        <IconTextButton
+          v-if="isUploadButtonEnabled"
+          type="accent"
+          size="md"
+          class="!h-10 [&>span]:hidden md:[&>span]:inline"
+          :label="$t('assetBrowser.uploadModel')"
+          :on-click="handleUploadClick"
+        >
+          <template #icon>
+            <i class="icon-[lucide--upload]" />
+          </template>
+        </IconTextButton>
+      </div>
     </template>
 
     <template #contentFilter>
@@ -52,9 +66,11 @@ import { useAsyncState } from '@vueuse/core'
 import { computed, provide, watch } from 'vue'
 import { useI18n } from 'vue-i18n'
 
+import IconTextButton from '@/components/button/IconTextButton.vue'
 import SearchBox from '@/components/input/SearchBox.vue'
 import BaseModalLayout from '@/components/widget/layout/BaseModalLayout.vue'
 import LeftSidePanel from '@/components/widget/panel/LeftSidePanel.vue'
+import { useFeatureFlags } from '@/composables/useFeatureFlags'
 import AssetFilterBar from '@/platform/assets/components/AssetFilterBar.vue'
 import AssetGrid from '@/platform/assets/components/AssetGrid.vue'
 import type { AssetDisplayItem } from '@/platform/assets/composables/useAssetBrowser'
@@ -168,4 +184,11 @@ function handleAssetSelectAndEmit(asset: AssetDisplayItem) {
   // It handles the appropriate transformation (filename extraction or full asset)
   props.onSelect?.(asset)
 }
+
+const { flags } = useFeatureFlags()
+const isUploadButtonEnabled = computed(() => flags.modelUploadButtonEnabled)
+
+function handleUploadClick() {
+  // Will be implemented in the future commit
+}
 </script>