add telemetry provider for cloud distribution (#6154)

## Summary

This code is entirely excluded from open-source, local, and desktop
builds. During minification and dead-code elimination, the Mixpanel
library is fully tree-shaken -- meaning no telemetry code is ever
included or downloaded in those builds. Even the inline callsites are
removed during the build (because `isCloud` becomes false and the entire
block becomes dead code and is removed). The code not only has no
effect, is not even distributed in the first place. We’ve gone to great
lengths to ensure this behavior.

Verification proof:


https://github.com/user-attachments/assets/b66c35f7-e233-447f-93da-4d70c433908d

Telemetry is *enabled only in the ComfyUI Cloud environment*. Its goal
is to help us understand and improve onboarding and new-user adoption.
ComfyUI aims to be accessible to everyone, but we know the learning
curve can be steep. Anonymous usage insights will help us identify where
users struggle and guide us toward making the experience more intuitive
and welcoming.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-6154-add-telemetry-provider-for-cloud-distribution-2926d73d3650813cb9ccfb3a2733848b)
by [Unito](https://www.unito.io)

---------

Co-authored-by: Claude <noreply@anthropic.com>

vite.config.mts

@@ -272,7 +272,8 @@ export default defineConfig({
     __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)
+    __BUILD_FLAGS__: JSON.stringify(BUILD_FLAGS),
+    __MIXPANEL_TOKEN__: JSON.stringify(process.env.MIXPANEL_TOKEN || '')
   },
 
   resolve: {