Fix setup port pollution, PostgreSQL sslmode fallback, and setup page behavior

[CatKeee]

Description

Changes

• Fix server.port pollution during setup

  • Prevent the frontend from injecting the browser port (e.g., 80/443) into backend configuration
  • The backend now determines the listening port based on its own startup configuration

• Add fallback for PostgreSQL sslmode=prefer

  • If the current driver does not support prefer, automatically fallback to sslmode=disable
  • Applies to both startup and setup database connections to prevent initialization failures

• Fix /setup page behavior

  • When the backend returns needs_setup=false, the frontend will:
    • Automatically redirect to the homepage
    • Prevent manual access to /setup
  • Avoids confusion where users think the system is not initialized

Notes

• No breaking changes
• Behavior is backward compatible with existing deployments