✨ [IMPROVEMENT] Allow configurable dev server port (default 9000 conflicts with Zscaler)

[puremcc]

Problem

APX's dev server defaults to port 9000, which conflicts with Zscaler Client Connector — a widely-deployed enterprise endpoint security agent that uses the same port for its local proxy listener.

In enterprises running Zscaler, apx dev fails out of the box because port 9000 is already bound. The current workaround requires each developer to manually edit their APX profile in ~/.apx/registry.toml to change the port per-app, which doesn't scale across teams and can't be checked into source control.

Request

Make the dev server port easily configurable at the project level (and/or CLI level), so teams can set it once and share it via version control. For example:

• A .apxconfig (or apx.config.json) file in the project root:

  {
    "dev": {
      "port": 9001
    }
  }

• And/or a CLI flag: apx dev --port 9001

Why this matters

• Zscaler is very common in regulated industries (finance, healthcare, government) — exactly the enterprises likely to adopt Databricks
• The per-user workaround is fragile, undiscoverable, and not version-controllable
• A project-level config is a small change that eliminates a real onboarding friction point

[renardeinside]

hey @puremcc fully agree with you but need deeper analysis on why our port detection logic doesn't detect that 9000 is already taken - I would classify it into two pieces:

1. improve port detection logic
2. allow users to customize dev server port explicitly via --port attribute on apx dev start

Both are totally valid, just need to finish the #139 first - might take a couple of weeks :(