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/components/UploadModelUpgradeModalFooter.vue

@@ -0,0 +1,34 @@
+<template>
+  <div class="flex flex-wrap justify-end gap-2 w-full">
+    <a
+      href="https://blog.comfy.org/p/comfy-cloud-new-features-and-pricing"
+      target="_blank"
+      rel="noopener noreferrer"
+      class="text-muted-foreground mr-auto underline flex items-center gap-2"
+    >
+      <i class="icon-[lucide--external-link]" />
+      <span>{{ $t('g.learnMore') }}</span>
+    </a>
+    <TextButton
+      :label="$t('g.close')"
+      type="transparent"
+      size="md"
+      @click="emit('close')"
+    />
+    <TextButton
+      :label="$t('subscription.required.subscribe')"
+      type="secondary"
+      size="md"
+      @click="emit('subscribe')"
+    />
+  </div>
+</template>
+
+<script setup lang="ts">
+import TextButton from '@/components/button/TextButton.vue'
+
+const emit = defineEmits<{
+  close: []
+  subscribe: []
+}>()
+</script>