Fix Windows error when setup port 8788 is already in use

[donvito]

Problem

On Windows, starting GAIA Personal Setup while another instance (or anything else) already owns 127.0.0.1:8788 failed with:

Error: listen tcp 127.0.0.1:8788: bind: Only one usage of each socket address ...

The message was also printed twice (cobra + our wrapper), which made a double-click launch look more broken than it was. Most often this happens when the user opens Setup a second time while the first window is still open.

Fix

• Probe /api/health before binding. If GAIA is already running, reopen the dashboard in the browser and exit cleanly instead of failing.
• If the port is busy with a non-GAIA process, return a clear, actionable error (open URL / close other window / use --addr).
• Bind before printing the “keep this window open” banner so failures are not preceded by success-looking text.
• Set SilenceErrors so “Error:” is only printed once.

Test plan

• go test ./...
• go vet ./...
• Manual: second wizard against a live GAIA listener → “Setup is already running” + exit 0
• Manual: foreign listener on same port → helpful port-busy error (single Error line)