feat: display and upload Civitai preview images in model upload flow (#7274)

## Summary
Stores and displays base64-encoded preview images from Civitai during
the model upload flow, uploading the preview as a separate asset linked
to the model.

## Changes
- **Schema**: Added `preview_image` field to `AssetMetadata` schema
- **Service**: Added `uploadAssetFromBase64` method to convert base64
data to blob and upload via FormData
- **Upload Flow**: Modified wizard to first upload preview image as
asset, then link it to model via `preview_id`
- **UI**: Display 56x56px preview thumbnail alongside model filename in
confirmation and success steps

## Review Focus
- Base64 to blob conversion and FormData upload implementation
- Sequential upload flow (preview first, then model with preview_id
reference)

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-7274-feat-display-and-upload-Civitai-preview-images-in-model-upload-flow-2c46d73d365081ff9b74c1791d23f6dd)
by [Unito](https://www.unito.io)

---------

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

src/platform/assets/services/assetService.ts

@@ -392,6 +392,59 @@ function createAssetService() {
     return await res.json()
   }
 
+  /**
+   * Uploads an asset from base64 data
+   *
+   * @param params - Upload parameters
+   * @param params.data - Base64 data URL (e.g., "data:image/png;base64,...")
+   * @param params.name - Display name (determines extension)
+   * @param params.tags - Optional freeform tags
+   * @param params.user_metadata - Optional custom metadata object
+   * @returns Promise<AssetItem & { created_new: boolean }> - Asset object with created_new flag
+   * @throws Error if upload fails
+   */
+  async function uploadAssetFromBase64(params: {
+    data: string
+    name: string
+    tags?: string[]
+    user_metadata?: Record<string, any>
+  }): Promise<AssetItem & { created_new: boolean }> {
+    // Validate that data is a data URL
+    if (!params.data || !params.data.startsWith('data:')) {
+      throw new Error(
+        'Invalid data URL: expected a string starting with "data:"'
+      )
+    }
+
+    // Convert base64 data URL to Blob
+    const blob = await fetch(params.data).then((r) => r.blob())
+
+    // Create FormData and append the blob
+    const formData = new FormData()
+    formData.append('file', blob, params.name)
+
+    if (params.tags) {
+      formData.append('tags', JSON.stringify(params.tags))
+    }
+
+    if (params.user_metadata) {
+      formData.append('user_metadata', JSON.stringify(params.user_metadata))
+    }
+
+    const res = await api.fetchApi(ASSETS_ENDPOINT, {
+      method: 'POST',
+      body: formData
+    })
+
+    if (!res.ok) {
+      throw new Error(
+        `Failed to upload asset from base64: ${res.status} ${res.statusText}`
+      )
+    }
+
+    return await res.json()
+  }
+
   return {
     getAssetModelFolders,
     getAssetModels,
@@ -402,7 +455,8 @@ function createAssetService() {
     deleteAsset,
     updateAsset,
     getAssetMetadata,
-    uploadAssetFromUrl
+    uploadAssetFromUrl,
+    uploadAssetFromBase64
   }
 }