Add distribution detection pattern (#6028)

## Summary

Establishes distribution-specific code pattern using compile-time
constants and dead code elimination. Demonstrates with Help Center by
hiding extension manager and update buttons in cloud distribution.

Below commentary makes assumption that minifcation and tree-shaking is
enabled (which isn't true yet, but will be eventually).

## Changes

- **What**: Added `src/platform/distribution/types.ts` with distribution
detection via `__DISTRIBUTION__` variable
- **Build**: Vite replaces `__DISTRIBUTION__` at build time using
environment variables
- **Tree-shaking**: All code not relevant to target distribution is
DCR'd and eliminated from bundle
- **Example**: Help Center hides "Manager Extension" menu item and
"Update" buttons in cloud builds

## Pattern

This PR defines a `__DISTRIBUTION__` variable which gets replaced at
build time by Vite using environment variables. All code not relevant to
the given distribution is then DCR'd and tree-shaken.

For simple cases (like this Help Center PR), import `isCloud` and use
compile-time conditionals:

```typescript
import { isCloud } from '@/platform/distribution/types'

if (!isCloud) {
  items.push({
    key: 'manager',
    action: async () => {
      await useManagerState().openManager({ ... })
    }
  })
}
```

The code is DCR'd at build time so there's zero runtime overhead - we
don't even incur the `if (isCloud)` cost because Terser eliminates it.

For complex services later, we'll add interfaces and use an index.ts
that exports different implementations under the same alias per
distribution. It will resemble a DI container but simpler since we don't
need runtime discovery like backend devs do. This guarantees types and
makes testing easier.

Example for services:
```typescript
// src/platform/storage/index.ts
import { isCloud } from '@/platform/distribution/types'

if (isCloud) {
  export { CloudStorage as StorageService } from './cloud'
} else {
  export { LocalStorage as StorageService } from './local'
}
```

Example for component variants:
```typescript
// src/components/downloads/index.ts
import { isCloud } from '@/platform/distribution/types'

if (isCloud) {
  export { default as DownloadButton } from './DownloadButton.cloud.vue'
} else {
  export { default as DownloadButton } from './DownloadButton.desktop.vue'
}
```

## Implementation Details

Distribution types (`src/platform/distribution/types.ts`):
```typescript
type Distribution = 'desktop' | 'localhost' | 'cloud'

declare global {
  const __DISTRIBUTION__: Distribution
}

const DISTRIBUTION: Distribution = __DISTRIBUTION__
export const isCloud = DISTRIBUTION === 'cloud'
```

Vite configuration adds the define:
```typescript
const DISTRIBUTION = (process.env.DISTRIBUTION || 'localhost') as
  | 'desktop'
  | 'localhost'
  | 'cloud'

export default defineConfig({
  define: {
    __DISTRIBUTION__: JSON.stringify(DISTRIBUTION)
  }
})
```

## Build Commands

```bash
pnpm build                      # localhost (default)
DISTRIBUTION=cloud pnpm build   # cloud
DISTRIBUTION=desktop pnpm build # desktop
```

## Future Applications

This pattern can be used with auth or telemetry services - which will
guarantee all the telemetry code, for example, is not even in the code
distributed in OSS Comfy whatsoever while still being able to develop
off `main`.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-6028-Add-distribution-detection-pattern-28a6d73d365081b08767d395472cd1bc)
by [Unito](https://www.unito.io)

vite.config.mts

@@ -24,6 +24,11 @@ const DISABLE_VUE_PLUGINS = process.env.DISABLE_VUE_PLUGINS === 'true'
 const DEV_SERVER_COMFYUI_URL =
   process.env.DEV_SERVER_COMFYUI_URL || 'http://127.0.0.1:8188'
 
+const DISTRIBUTION = (process.env.DISTRIBUTION || 'localhost') as
+  | 'desktop'
+  | 'localhost'
+  | 'cloud'
+
 export default defineConfig({
   base: '',
   server: {
@@ -195,7 +200,8 @@ export default defineConfig({
     __SENTRY_DSN__: JSON.stringify(process.env.SENTRY_DSN || ''),
     __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'
+    __USE_PROD_CONFIG__: process.env.USE_PROD_CONFIG === 'true',
+    __DISTRIBUTION__: JSON.stringify(DISTRIBUTION)
   },
 
   resolve: {