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/composables/useFeatureFlags.ts

@@ -11,7 +11,8 @@ export enum ServerFeatureFlag {
   MAX_UPLOAD_SIZE = 'max_upload_size',
   MANAGER_SUPPORTS_V4 = 'extension.manager.supports_v4',
   MODEL_UPLOAD_BUTTON_ENABLED = 'model_upload_button_enabled',
-  ASSET_UPDATE_OPTIONS_ENABLED = 'asset_update_options_enabled'
+  ASSET_UPDATE_OPTIONS_ENABLED = 'asset_update_options_enabled',
+  PRIVATE_MODELS_ENABLED = 'private_models_enabled'
 }
 
 /**
@@ -47,6 +48,13 @@ export function useFeatureFlags() {
           false
         )
       )
+    },
+    get privateModelsEnabled() {
+      // Check remote config first (from /api/features), fall back to websocket feature flags
+      return (
+        remoteConfig.value.private_models_enabled ??
+        api.getServerFeature(ServerFeatureFlag.PRIVATE_MODELS_ENABLED, false)
+      )
     }
   })