feat: add React frontend with login, signup, dashboard pages and admin seeding support

backend/sample.config.yaml

@@ -19,6 +19,11 @@ auth:
   cookie_name: "zhealth_session"
   cookie_secure: false  # Set to true in production with HTTPS
 
+# Default admin user (created on first startup)
+admin:
+  username: "admin"
+  password: "${ADMIN_PASSWORD}"  # Use env var in production
+
 ai:
   provider: "gemini"  # Options: gemini | openai | anthropic
   model: "gemini-3-flash-preview"