[backport rh-test] make 'require subscription' toggleable (#6147)

## Summary

Backport of PR #6144 to the `rh-test` branch.

This adds build time feature flags system starting with a flag that
indicates whether subscription is required to use the app. This is only
used on cloud.

## Changes

- Added build feature flags system via `__BUILD_FLAGS__` global
- Added `REQUIRE_SUBSCRIPTION` flag that can be set via environment
variable
- Conditionally load subscription-related components based on the flag
- Updated run button logic to respect the subscription requirement flag
- Updated settings UI to show subscription panel only when required

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-6147-Backport-make-require-subscription-toggleable-in-build-to-rh-test-2916d73d365081e89bcfc4502315a812)
by [Unito](https://www.unito.io)

src/components/actionbar/ComfyRunButton/index.ts

@@ -2,6 +2,6 @@ import { defineAsyncComponent } from 'vue'
 
 import { isCloud } from '@/platform/distribution/types'
 
-export default isCloud
+export default isCloud && __BUILD_FLAGS__.REQUIRE_SUBSCRIPTION
   ? defineAsyncComponent(() => import('./CloudRunButtonWrapper.vue'))
   : defineAsyncComponent(() => import('./ComfyQueueButton.vue'))