Allow single-label hostnames as a Host target (#126)

The target ("ip") field validator required at least two dot-separated
labels, rejecting legitimate single-label hostnames like Docker Compose
service names ("sso-manager"), /etc/hosts entries, or anything resolved
via a search domain. This was enforced identically client-side
(public/lib/js/val.js) and server-side (utils/hostname_validate.js,
routes/host.js), so there was no way to set one through the UI or API
— only by writing to the Host model directly, bypassing validation
entirely (which is how theta-env's setup.sh registers sso-manager as a
target today, working only because it calls Host.create() directly).

Relax HOSTNAME in both places to accept either a bare single label or
the existing dotted-FQDN pattern. Flips the one existing test that
codified the old behavior (isValidHostname('localhost') was asserted
false) and adds coverage for the reported case.

nodejs/views/hosts.ejs

@@ -489,7 +489,7 @@
 
 							<div class="form-group">
 								<label for="ip" class="form-label">Target IP or host name</label>
-								<input type="text" name="ip" class="form-control" placeholder="ex: 10.10.10.10 or app.internal.net" validate="target:3" />
+								<input type="text" name="ip" class="form-control" placeholder="ex: 10.10.10.10, app.internal.net, or sso-manager" validate="target:3" />
 								<b class="invalid-feedback"></b>
 								<small class="field-help text-muted d-block">Where matching requests are proxied. Hostname or IP only &mdash; no protocol, port, or path.</small>
 							</div>