gc dashboard doesn't work — API errors even with correct config

[csells]

Bug

gc dashboard serve starts the web server but shows no data. Every API call fails with errors, even when the API URL is correctly specified.

With bare port (additional papercut):

gc dashboard serve --api 9443
# → unsupported protocol scheme ""

With full URL:

gc dashboard serve --api http://127.0.0.1:9443
# → connection refused or similar errors on all /v0/* endpoints

The dashboard starts and is accessible at http://localhost:8080 but displays nothing because the API server it depends on is not serving data.

Expected behavior

gc dashboard should either:

1. Work end-to-end when a controller is running with [api] configured
2. Or fail fast with a clear error if the API server isn't reachable, rather than starting an empty dashboard that silently logs errors

Steps to reproduce

1. Add [api] with port = 9443 to city.toml
2. Start the city controller
3. Run gc dashboard serve --api http://127.0.0.1:9443
4. Open http://localhost:8080 — no data shown, errors in terminal

Notes

Related to #431 (dashboard should work out of the box). The dashboard currently has multiple barriers: no API by default, no auto-discovery, bare port accepted silently, and even when configured it doesn't display data.