fix(site/src/pages/TemplateBuilder): block continuing without base template or module selection (#26626)

Fixes DEVEX-520

The Template Builder wizard allowed users to continue past the base
template selection step without selecting a template, and past the
module selection step without selecting any modules.

Adds `canContinue` validation for the `base-infra` and
`module-select` steps in `computeCanContinue()`. The Continue button
is now disabled until a base template is selected on the first step
and at least one module is selected on the module selection step.

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site/src/pages/TemplateBuilder/TemplateBuilderPageView.tsx

@@ -254,6 +254,8 @@ function computeCanContinue(
 	moduleVarMap: Record<string, Record<string, string>>,
 ): boolean {
 	switch (stepId) {
+		case "base-infra":
+			return state.selectedBase !== null;
 		case "base-parameters":
 			return baseParametersComplete(
 				basesData,