[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/UploadModelDialog.vue

@@ -4,7 +4,13 @@
   >
     <!-- Step 1: Enter URL -->
     <UploadModelUrlInput
-      v-if="currentStep === 1"
+      v-if="currentStep === 1 && flags.huggingfaceModelImportEnabled"
+      v-model="wizardData.url"
+      :error="uploadError"
+      class="flex-1"
+    />
+    <UploadModelUrlInputCivitai
+      v-else-if="currentStep === 1"
       v-model="wizardData.url"
       :error="uploadError"
     />
@@ -46,14 +52,17 @@
 <script setup lang="ts">
 import { onMounted } from 'vue'
 
+import { useFeatureFlags } from '@/composables/useFeatureFlags'
 import UploadModelConfirmation from '@/platform/assets/components/UploadModelConfirmation.vue'
 import UploadModelFooter from '@/platform/assets/components/UploadModelFooter.vue'
 import UploadModelProgress from '@/platform/assets/components/UploadModelProgress.vue'
 import UploadModelUrlInput from '@/platform/assets/components/UploadModelUrlInput.vue'
+import UploadModelUrlInputCivitai from '@/platform/assets/components/UploadModelUrlInputCivitai.vue'
 import { useModelTypes } from '@/platform/assets/composables/useModelTypes'
 import { useUploadModelWizard } from '@/platform/assets/composables/useUploadModelWizard'
 import { useDialogStore } from '@/stores/dialogStore'
 
+const { flags } = useFeatureFlags()
 const dialogStore = useDialogStore()
 const { modelTypes, fetchModelTypes } = useModelTypes()