feat: add upgrade modal for model upload when private models disabled (#7124)

## Summary
Adds a dedicated upgrade modal that appears when users without private
models access try to upload models, providing a clear path to upgrade
their subscription.

## Changes
- **New upgrade modal**: Created `UploadModelUpgradeModal` with
dedicated body, header, and footer components
- **Conditional rendering**: Modified `AssetBrowserModal` to show
upgrade modal when `privateModelsEnabled` flag is false
- **Subscription integration**: Connected upgrade flow to existing
subscription system via `showSubscriptionDialog()`
- **Localization**: Added localization keys for upgrade messaging

## Review Focus
- Conditional logic in `AssetBrowserModal.handleUploadClick()` based on
feature flags
- Component naming consistency (all upgrade-related components prefixed
with `UploadModelUpgrade`)
- Footer component refactoring maintains existing upload wizard behavior

🤖 Generated with [Claude Code](https://claude.com/claude-code)

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-7124-feat-add-upgrade-modal-for-model-upload-when-private-models-disabled-2be6d73d36508147b72eea8a1d6ab772)
by [Unito](https://www.unito.io)

---------

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Alexander Brown <drjkl@comfy.org>

src/platform/assets/composables/useModelUpload.ts

@@ -1,6 +1,8 @@
 import { useFeatureFlags } from '@/composables/useFeatureFlags'
 import UploadModelDialog from '@/platform/assets/components/UploadModelDialog.vue'
 import UploadModelDialogHeader from '@/platform/assets/components/UploadModelDialogHeader.vue'
+import UploadModelUpgradeModal from '@/platform/assets/components/UploadModelUpgradeModal.vue'
+import UploadModelUpgradeModalHeader from '@/platform/assets/components/UploadModelUpgradeModalHeader.vue'
 import type { AssetItem } from '@/platform/assets/schemas/assetSchema'
 import { useDialogStore } from '@/stores/dialogStore'
 import type { UseAsyncStateReturn } from '@vueuse/core'
@@ -14,22 +16,38 @@ export function useModelUpload(
   const isUploadButtonEnabled = computed(() => flags.modelUploadButtonEnabled)
 
   function showUploadDialog() {
-    dialogStore.showDialog({
-      key: 'upload-model',
-      headerComponent: UploadModelDialogHeader,
-      component: UploadModelDialog,
-      props: {
-        onUploadSuccess: async () => {
-          await execute?.()
+    if (!flags.privateModelsEnabled) {
+      // Show upgrade modal if private models are disabled
+      dialogStore.showDialog({
+        key: 'upload-model-upgrade',
+        headerComponent: UploadModelUpgradeModalHeader,
+        component: UploadModelUpgradeModal,
+        dialogComponentProps: {
+          pt: {
+            header: 'py-0! pl-0!',
+            content: 'p-0!'
+          }
         }
-      },
-      dialogComponentProps: {
-        pt: {
-          header: 'py-0! pl-0!',
-          content: 'p-0!'
+      })
+    } else {
+      // Show regular upload modal
+      dialogStore.showDialog({
+        key: 'upload-model',
+        headerComponent: UploadModelDialogHeader,
+        component: UploadModelDialog,
+        props: {
+          onUploadSuccess: async () => {
+            await execute?.()
+          }
+        },
+        dialogComponentProps: {
+          pt: {
+            header: 'py-0! pl-0!',
+            content: 'p-0!'
+          }
         }
-      }
-    })
+      })
+    }
   }
   return { isUploadButtonEnabled, showUploadDialog }
 }