change cloud feature flags to be loaded dynamically at runtime rather than set in build (#6246)

## Summary

Implements server-side remote configuration to decouple runtime behavior
from build artifacts, enabling dynamic configuration updates without
redeployment.

## Technical Changes

- **Replaced** build-time constants (`__MIXPANEL_TOKEN__`,
`__BUILD_FLAGS__`) with runtime configuration loaded from
`/api/features`
- Configuration now sourced from `window.__CONFIG__` (hydrated from
`/api/features` endpoint)
- **Added** `src/platform/remoteConfig/` service that polls server
configuration every 30 seconds
- **Modified** application bootstrap sequence in `main.ts` to load
remote config before module initialization (required for cloud builds)
- **Removed** global constants: `__BUILD_FLAGS__`, `__MIXPANEL_TOKEN__`.
Runtime subscription enforcement toggle via `subscription_required` flag
- Server health alerts with variant-based severity rendering
(info/warning/error) via topbar badges

## Rationale

- **Build-once-deploy-anywhere**: Single immutable artifact promoted
through environments (staging → production)
- **Zero-downtime configuration**: Update behavior without rebuilding or
redeploying the application
- **Incident response**: Disable features or display alerts dynamically
in response to outages or degraded service
- **Instant rollback**: Revert configuration changes server-side without
artifact redeployment
- **Progressive delivery**: Enable A/B testing, canary releases, and
user/region-based configuration
- **Environment parity**: Eliminate configuration drift between staging
and production builds
- Decouples deployment cadence from configuration changes
- Enables GitOps workflows for configuration management separate from
code deployments
- Supports real-time operational control of client behavior
- Reduces build matrix complexity (no per-environment builds)

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-6246-change-cloud-feature-flags-to-be-loaded-dynamically-at-runtime-rather-than-set-in-build-2966d73d3650811cbb41c9093961037a)
by [Unito](https://www.unito.io)

vite.config.mts

@@ -53,10 +53,6 @@ const DEV_SERVER_COMFYUI_URL =
 const cloudProxyConfig =
   DISTRIBUTION === 'cloud' ? { secure: false, changeOrigin: true } : {}
 
-const BUILD_FLAGS = {
-  REQUIRE_SUBSCRIPTION: process.env.REQUIRE_SUBSCRIPTION === 'true'
-}
-
 export default defineConfig({
   base: '',
   server: {
@@ -306,9 +302,7 @@ export default defineConfig({
     __ALGOLIA_APP_ID__: JSON.stringify(process.env.ALGOLIA_APP_ID || ''),
     __ALGOLIA_API_KEY__: JSON.stringify(process.env.ALGOLIA_API_KEY || ''),
     __USE_PROD_CONFIG__: process.env.USE_PROD_CONFIG === 'true',
-    __DISTRIBUTION__: JSON.stringify(DISTRIBUTION),
-    __BUILD_FLAGS__: JSON.stringify(BUILD_FLAGS),
-    __MIXPANEL_TOKEN__: JSON.stringify(process.env.MIXPANEL_TOKEN || '')
+    __DISTRIBUTION__: JSON.stringify(DISTRIBUTION)
   },
 
   resolve: {