Merge selected environment config into __CONF__

[rzueger]

Summary

• Keys defined under projectConf.environments.<env> now override the top-level config before it is packinized into __CONF__.
• Clients can read env-specific flags via __CONF__.<key> directly.
• Enables per-environment feature toggles without introducing new webpack globals.

Motivation

Today webpack already picks the per-environment block (test vs production) to derive the Firebase globals (__FIREBASE_PROJECT_ID__, etc.). But everything else in __CONF__ is project-wide. If a project wants an opt-in flag that is true in test and false in prod (or vice versa), the only way is another top-level webpack global — which doesn't scale.

With this change, a project can define under environments.test (or environments.production) any key that is also accepted at the top level, and the selected environment's value wins at build time.

Test plan

• npm test — all 1806 tests pass
• npm run typecheck clean
• npm run build --project=<any> produces a working bundle
• No observable change for any existing project — no env block currently defines keys that collide with top-level config