[feat] Add HuggingFace model import support (#7540)

## Summary
Adds HuggingFace as a model import source alongside CivitAI, with
improved UX for model type selection and UTF-8 filename support.

## Changes
- **Import Sources**: Implemented extensible import source handler
pattern supporting both CivitAI and HuggingFace
- **UTF-8 Support**: Decode URL-encoded filenames to properly display
international characters (e.g., Chinese)
- **UX**: Sort model types alphabetically for easier selection
- **Feature Flag**: Added `huggingfaceModelImportEnabled` flag for
gradual rollout
- **i18n**: Use proper template parameters for localized error messages

## Technical Details
- Created `ImportSourceHandler` interface for extensibility
- Refactored existing CivitAI logic into handler pattern
- Added URL validation per source
- Filename decoding handles malformed URLs gracefully

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-7540-feat-Add-HuggingFace-model-import-support-2cb6d73d3650818f966cca89244e8c36)
by [Unito](https://www.unito.io)

---------

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: GitHub Action <action@github.com>

src/platform/assets/components/UploadModelDialogHeader.vue

@@ -1,7 +1,11 @@
 <template>
   <div class="flex items-center gap-2 p-4 font-bold">
-    <img src="/assets/images/civitai.svg" class="size-4" />
-    <span>{{ $t('assetBrowser.uploadModelFromCivitai') }}</span>
+    <img
+      v-if="!flags.huggingfaceModelImportEnabled"
+      src="/assets/images/civitai.svg"
+      class="size-4"
+    />
+    <span>{{ $t(titleKey) }}</span>
     <span
       class="rounded-full bg-white px-1.5 py-0 text-xxs font-inter font-semibold uppercase text-black"
     >
@@ -9,3 +13,17 @@
     </span>
   </div>
 </template>
+
+<script setup lang="ts">
+import { computed } from 'vue'
+
+import { useFeatureFlags } from '@/composables/useFeatureFlags'
+
+const { flags } = useFeatureFlags()
+
+const titleKey = computed(() => {
+  return flags.huggingfaceModelImportEnabled
+    ? 'assetBrowser.uploadModelGeneric'
+    : 'assetBrowser.uploadModelFromCivitai'
+})
+</script>