feat: add isNightly build flag for nightly-only features (#8149)

## Summary

Adds a compile-time `__IS_NIGHTLY__` constant that detects whether the
build is from the main branch (nightly) or a core/* branch (RC/stable).
The detection logic in vite.config.mts auto-detects based on
`GITHUB_REF_NAME === 'main'` in CI, with explicit override support via
`IS_NIGHTLY` environment variable. Exports `isNightly` from
`src/platform/distribution/types.ts` for use throughout the codebase.
Includes unit tests for the detection logic.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8149-feat-add-isNightly-build-flag-for-nightly-only-features-2ec6d73d365081c09930edec1c6644f5)
by [Unito](https://www.unito.io)

scripts/vite-define-shim.ts

@@ -12,6 +12,7 @@ declare global {
   const __ALGOLIA_API_KEY__: string
   const __USE_PROD_CONFIG__: boolean
   const __DISTRIBUTION__: 'desktop' | 'localhost' | 'cloud'
+  const __IS_NIGHTLY__: boolean
 }
 
 type GlobalWithDefines = typeof globalThis & {
@@ -22,6 +23,7 @@ type GlobalWithDefines = typeof globalThis & {
   __ALGOLIA_API_KEY__: string
   __USE_PROD_CONFIG__: boolean
   __DISTRIBUTION__: 'desktop' | 'localhost' | 'cloud'
+  __IS_NIGHTLY__: boolean
   window?: Record<string, unknown>
 }
 
@@ -36,6 +38,7 @@ globalWithDefines.__ALGOLIA_APP_ID__ = ''
 globalWithDefines.__ALGOLIA_API_KEY__ = ''
 globalWithDefines.__USE_PROD_CONFIG__ = false
 globalWithDefines.__DISTRIBUTION__ = 'localhost'
+globalWithDefines.__IS_NIGHTLY__ = false
 
 // Provide a minimal window shim for Node environment
 // This is needed for code that checks window existence during imports