docs: add .env.example files for client and server

apps/client/.env.example

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+# Base URL of the CalChat API server
+# Must include the /api path suffix
+# Use your local network IP for mobile device testing, or localhost for emulator/web
+# Examples:
+#   http://192.168.178.22:3001/api   (local network, for physical device)
+#   http://localhost:3001/api         (emulator or web)
+#   https://calchat.example.com/api   (production)
+EXPO_PUBLIC_API_URL=http://localhost:3001/api

apps/server/.env.example

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+# OpenAI API key for GPT-based chat assistant
+# Required for AI chat functionality
+OPENAI_API_KEY=sk-proj-your-key-here
+
+# Port the server listens on
+# Default: 3000
+PORT=3000
+
+# MongoDB connection URI
+# Default: mongodb://localhost:27017/calchat
+# The Docker Compose setup uses root:mongoose credentials with authSource=admin
+MONGODB_URI=mongodb://root:mongoose@localhost:27017/calchat?authSource=admin
+
+# Use static test responses instead of real GPT calls
+# Values: true | false
+# Default: false
+USE_TEST_RESPONSES=false
+
+# Log level for pino logger
+# Values: debug | info | warn | error | fatal
+# Default: debug (development), info (production)
+LOG_LEVEL=debug
+
+# Node environment
+# Values: development | production
+# development = pretty-printed logs, production = JSON logs
+# Default: development
+NODE_ENV=development