Allow configuring the desktop dev remote-debugging port

Title:
Allow configuring the desktop dev remote-debugging port

Description:
packages/desktop/src/main/index.ts calls
app.commandLine.appendSwitch("remote-debugging-port", "9222") in unpackaged
runs, hard-coded. When the developer already has something on 9222 (VS Code,
another Electron app, a second opencode desktop dev build), the DevTools port
silently fails to bind. There is no env-var override today.

Plugins: n/a
OpenCode version: dev (dc4449df0d)

Steps to reproduce:

1. Start anything that listens on 9222 (e.g. bun --inspect=9222).
2. bun run dev in packages/desktop.
3. The DevTools port is taken; the second instance fails to expose the
   inspector.

Expected behaviour:

• An env var (OPENCODE_REMOTE_DEBUGGING_PORT) selects the port.
• A default value (e.g. 9223, one above the OS-default 9222) keeps current
  behaviour when nothing is on 9222.

Actual behaviour:

• Port is hard-coded, no override, build script does not even log a warning.

Operating System: n/a
Terminal: n/a

Related / duplicate check: no related upstream issue found.