feat(frontend): configure staging and production environments (FRO-61)

- Add .env.staging with Railway staging API URL
- Add .env.production placeholder for future production environment
- Add build:staging script to use staging configuration
- Allow .env.staging and .env.production in git (no secrets, only public URLs)
- Simplify api.ts to rely on environment files

Environment configuration:
- Development: localhost:5000/api (fallback)
- Staging: https://fantasyrealm-api-staging.up.railway.app/api
- Production: To be configured with Railway production URL

Build: ✓ (staging mode verified)

Author: pierrick-fonquerne

.gitignore

@@ -113,6 +113,8 @@ pids/
 .env
 .env.*
 !.env.example
+!.env.staging
+!.env.production
 
 # =====================================================
 # IDEs and Editors

src/frontend/.env.production

@@ -0,0 +1,2 @@
+# Production environment - Railway backend (à configurer quand l'environnement de production sera créé)
+# VITE_API_URL=https://fantasyrealm-api-production.up.railway.app/api

src/frontend/.env.staging

@@ -0,0 +1,2 @@
+# Staging environment - Railway backend
+VITE_API_URL=https://fantasyrealm-api-staging.up.railway.app/api

src/frontend/package.json

@@ -6,6 +6,7 @@
   "scripts": {
     "dev": "vite",
     "build": "tsc -b && vite build",
+    "build:staging": "tsc -b && vite build --mode staging",
     "lint": "eslint .",
     "lint:fix": "eslint . --fix",
     "format": "prettier --write \"src/**/*.{ts,tsx,css}\"",