v0.27.0 — user-configurable app subdomains

Users can now customize the subdomain for each app during deployment
instead of using a fixed value. The deploy page shows an editable text
input with the default pre-filled and the base domain as a suffix.

New "subdomain" deploy field type with DNS-safe format validation,
reserved name blocklist, and uniqueness check across deployed stacks.
Locked after deploy — changing requires Remove + Redeploy.

Backward compatible: InjectMissingFields() auto-fills SUBDOMAIN from
.felhom.yml defaults for existing deployed apps on next sync/restart.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

controller/internal/web/templates/style.css

@@ -574,6 +574,13 @@ select.form-control { appearance: auto; }
 select.form-control option { background: var(--bg-secondary); color: var(--text-primary); }
 .input-with-button { display: flex; gap: .5rem; }
 .input-with-button .form-control { flex: 1; }
+.subdomain-input-group { display: flex; align-items: center; }
+.subdomain-input-group .subdomain-input { max-width: 14rem; border-top-right-radius: 0; border-bottom-right-radius: 0; border-right: none; }
+.subdomain-input-group .subdomain-suffix {
+    padding: .55rem .75rem; background: var(--bg-primary); border: 1px solid var(--border-color);
+    border-left: none; border-radius: 0 8px 8px 0; color: var(--text-secondary);
+    font-family: var(--font-mono, monospace); font-size: .9rem; white-space: nowrap;
+}
 .form-hint { display: block; font-size: .8rem; color: var(--text-muted); margin-top: .25rem; }
 .required { color: var(--red); }
 .locked-hint { font-size: .75rem; color: var(--text-muted); font-weight: 400; margin-left: .5rem; }