Fail fast when required Firebase environment variables are missing

[MoerAI]

Summary

Firebase config silently falls back to empty strings when required NEXT_PUBLIC_FIREBASE_* environment variables are missing.

Evidence

• src/lib/firebase/config.ts:7-12 uses process.env.NEXT_PUBLIC_FIREBASE_* || '' for all Firebase config values.
• .env.example documents the required variables as placeholders.

Impact

A deployment with missing environment variables can fail later with confusing Firebase runtime errors instead of failing fast with a clear configuration message. This is not a secret exposure issue, but it weakens operational reliability.

Minimal Fix

• Validate all required NEXT_PUBLIC_FIREBASE_* values before initializing Firebase.
• Throw a clear error naming the missing variable(s).
• Keep .env.local ignored and continue committing only .env.example placeholders.

Acceptance Criteria

• Missing required Firebase env vars produce a clear error during local startup/build/runtime initialization.
• Valid env vars initialize Firebase exactly as before.
• No real keys are committed.