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)

src/platform/distribution/types.ts

@@ -9,6 +9,7 @@ type Distribution = 'desktop' | 'localhost' | 'cloud'
 
 declare global {
   const __DISTRIBUTION__: Distribution
+  const __IS_NIGHTLY__: boolean
 }
 
 /** Current distribution - replaced at compile time */
@@ -18,3 +19,10 @@ const DISTRIBUTION: Distribution = __DISTRIBUTION__
 export const isDesktop = DISTRIBUTION === 'desktop' || isElectron() // TODO: replace with build var
 export const isCloud = DISTRIBUTION === 'cloud'
 // export const isLocalhost = DISTRIBUTION === 'localhost' || (!isDesktop && !isCloud)
+
+/**
+ * Whether this is a nightly build (from main branch).
+ * Nightly builds may show experimental features and surveys.
+ * @public
+ */
+export const isNightly = __IS_NIGHTLY__