feat(site): warn about active prebuilds when duplicating template (#22945)

## Description

When duplicating a template that has prebuilds configured, a warning
alert is now shown above the create template form. The warning displays
the total number of prebuilds that will be automatically created.

![Warning
example](https://img.shields.io/badge/⚠️-This_template_has_prebuilds_configured-orange)

### Changes

**Single file modified:**
`site/src/pages/CreateTemplatePage/DuplicateTemplateView.tsx`

- Fetches presets for the template's active version using the existing
`templateVersionPresets` React Query helper
- Computes total prebuild count by summing `DesiredPrebuildInstances`
across all presets
- Renders a warning `<Alert>` above the form when prebuilds are
configured

### Design decisions

| Decision | Rationale |
|---|---|
| Warning in `DuplicateTemplateView`, not `CreateTemplateForm` | Only
the duplicate flow needs this. Keeps data fetching local. No new props.
|
| Feature-flag gated (`workspace_prebuilds`) | Matches existing pattern
in `TemplateLayout.tsx`. |
| Non-blocking query | Presets fetch failure shouldn't prevent
duplication. Warning is informational. |
| Count with pluralization | Users know exactly how many prebuilds will
spin up. |

<img width="1136" height="375" alt="image"
src="https://github.com/user-attachments/assets/1ca42608-a204-48f5-b27d-6d476ab32fa7"
/>


Closes #18987

site/src/pages/CreateTemplatePage/DuplicateTemplateView.tsx

@@ -3,13 +3,16 @@ import {
 	template,
 	templateVersion,
 	templateVersionLogs,
+	templateVersionPresets,
 	templateVersionVariables,
 } from "api/queries/templates";
 import type { Template, TemplateVersion } from "api/typesGenerated";
 import { useDashboard } from "modules/dashboard/useDashboard";
+import { useFeatureVisibility } from "modules/dashboard/useFeatureVisibility";
 import type { FC } from "react";
 import { useQuery } from "react-query";
 import { useNavigate, useSearchParams } from "react-router";
+import { Alert } from "#/components/Alert/Alert";
 import { ErrorAlert } from "#/components/Alert/ErrorAlert";
 import { Loader } from "#/components/Loader/Loader";
 import { CreateTemplateForm } from "./CreateTemplateForm";
@@ -38,6 +41,16 @@ export const DuplicateTemplateView: FC<CreateTemplatePageViewProps> = ({
 		...templateVersionVariables(activeVersionId),
 		enabled: templateQuery.isSuccess,
 	});
+	const { workspace_prebuilds: prebuildsEnabled } = useFeatureVisibility();
+	const presetsQuery = useQuery({
+		...templateVersionPresets(activeVersionId),
+		enabled: templateQuery.isSuccess && prebuildsEnabled,
+	});
+	const totalPrebuilds =
+		presetsQuery.data?.reduce(
+			(sum, preset) => sum + (preset.DesiredPrebuildInstances ?? 0),
+			0,
+		) ?? 0;
 	const isLoading =
 		templateQuery.isLoading ||
 		templateVersionQuery.isLoading ||
@@ -64,29 +77,38 @@ export const DuplicateTemplateView: FC<CreateTemplatePageViewProps> = ({
 	}
 
 	return (
-		<CreateTemplateForm
-			{...formPermissions}
-			variablesSectionRef={variablesSectionRef}
-			onOpenBuildLogsDrawer={onOpenBuildLogsDrawer}
-			copiedTemplate={templateQuery.data as Template}
-			error={error}
-			isSubmitting={isCreating}
-			variables={templateVersionVariablesQuery.data}
-			onCancel={() => navigate(-1)}
-			jobError={isJobError ? error.job.error : undefined}
-			logs={templateVersionLogsQuery.data}
-			onSubmit={async (formData) => {
-				await onCreateTemplate({
-					organization: (templateQuery.data as Template).organization_name,
-					version: firstVersionFromFile(
-						(templateVersionQuery.data as TemplateVersion).job.file_id,
-						formData.user_variable_values,
-						formData.provisioner_type,
-						formData.tags,
-					),
-					template: newTemplate(formData),
-				});
-			}}
-		/>
+		<>
+			{totalPrebuilds > 0 && (
+				<Alert severity="warning" css={{ marginBottom: 16 }}>
+					This template has prebuilds configured. Duplicating this template will
+					automatically cause {totalPrebuilds}{" "}
+					{totalPrebuilds === 1 ? "prebuild" : "prebuilds"} to be created.
+				</Alert>
+			)}
+			<CreateTemplateForm
+				{...formPermissions}
+				variablesSectionRef={variablesSectionRef}
+				onOpenBuildLogsDrawer={onOpenBuildLogsDrawer}
+				copiedTemplate={templateQuery.data as Template}
+				error={error}
+				isSubmitting={isCreating}
+				variables={templateVersionVariablesQuery.data}
+				onCancel={() => navigate(-1)}
+				jobError={isJobError ? error.job.error : undefined}
+				logs={templateVersionLogsQuery.data}
+				onSubmit={async (formData) => {
+					await onCreateTemplate({
+						organization: (templateQuery.data as Template).organization_name,
+						version: firstVersionFromFile(
+							(templateVersionQuery.data as TemplateVersion).job.file_id,
+							formData.user_variable_values,
+							formData.provisioner_type,
+							formData.tags,
+						),
+						template: newTemplate(formData),
+					});
+				}}
+			/>
+		</>
 	);
 };