refactor(k8s): keep the big-demo theme as real files under infra/keycloak/themes (refs #177)

The theme was inline text in a Helm template. It now lives next to the realms
and is seeded as rr-kc-theme by seed-configmaps.sh, like every other file
input, so it can be edited as a normal Keycloak theme. demo.otpAutofill now
only decides whether KC_SPI_THEME_DEFAULT is set (big.env skips values that
render empty); off, Keycloak keeps its stock theme.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

infra/helm/big-reference/templates/_helpers.tpl

@@ -129,13 +129,17 @@ volumes:
 {{/*
 Env list from a map. Every value is run through `tpl`, so values.yaml can name
 cluster-internal hosts ({{ .Release.Namespace }}) and the node address
-({{ .Values.host }}) without the chart hard-coding either.
+({{ .Values.host }}) without the chart hard-coding either. A value that renders
+empty is left out, which is how a setting is made conditional on a chart value.
 */}}
 {{- define "big.env" -}}
 {{- $root := index . 0 -}}
 {{- range $k, $v := index . 1 }}
+{{- $val := tpl (toString $v) $root }}
+{{- if $val }}
 - name: {{ $k }}
-  value: {{ tpl (toString $v) $root | quote }}
+  value: {{ $val | quote }}
+{{- end }}
 {{- end }}
 {{- end -}}