Make backend API URL configurable via environment variables

Updated frontend to use BACKEND_API_URL environment variable at compile time with fallback to localhost. Added configuration to Trunk.toml and .env.example for development.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

.env.example

@@ -9,5 +9,9 @@ CALDAV_CALENDAR_PATH=/calendars/your-username/personal/
 # Optional: Task/Todo collection path
 CALDAV_TASKS_PATH=/calendars/your-username/tasks/
 
+# Backend API Configuration
+# Set this to point to your backend API server
+BACKEND_API_URL=http://localhost:3000/api
+
 # Development settings
 RUST_LOG=info