feat(site): add BaseInfraSelectStep (#26424)

Implement the first wizard step for selecting a base infrastructure
template.

- Add `getTemplateBuilderBases` API client method and react-query
wrapper
- Render a responsive grid of `TemplateCard` components fetched from
`GET /api/v2/templatebuilder/bases`
- Map `TemplateBuilderBase` to `SelectedBaseMeta` with `hasParameters`
derived from the base variables
- Single-choice selection that dispatches `SET_BASE` to wizard state,
preserving selection on back navigation

Relates to [DEVEX-283](https://linear.app/codercom/issue/DEVEX-283).

> [!NOTE]
> This PR was authored with Coder Agents.

site/src/api/api.ts

@@ -2446,6 +2446,12 @@ class ApiMethods {
 		return response.data;
 	};
 
+	getTemplateBuilderBases =
+		async (): Promise<TypesGen.TemplateBuilderBasesResponse> => {
+			const response = await this.axios.get("/api/v2/templatebuilder/bases");
+			return response.data;
+		};
+
 	uploadFile = async (file: File): Promise<TypesGen.UploadResponse> => {
 		const response = await this.axios.post("/api/v2/files", file, {
 			headers: { "Content-Type": file.type },

site/src/api/queries/templateBuilder.ts

@@ -0,0 +1,6 @@
+import { API } from "#/api/api";
+
+export const templateBuilderBases = () => ({
+	queryKey: ["templateBuilder", "bases"],
+	queryFn: API.getTemplateBuilderBases,
+});

site/src/pages/TemplateBuilder/BaseInfraSelectStep.tsx

@@ -0,0 +1,66 @@
+import type { FC } from "react";
+import { useQuery } from "react-query";
+import { templateBuilderBases } from "#/api/queries/templateBuilder";
+import type { TemplateBuilderBase } from "#/api/typesGenerated";
+import { ErrorAlert } from "#/components/Alert/ErrorAlert";
+import { Loader } from "#/components/Loader/Loader";
+import { TemplateCard } from "./TemplateCard";
+import type { SelectedBaseMeta } from "./wizardState";
+
+interface BaseInfraSelectStepProps {
+	selectedBaseId: string | null;
+	onSelectBase: (base: SelectedBaseMeta) => void;
+}
+
+function toSelectedBaseMeta(base: TemplateBuilderBase): SelectedBaseMeta {
+	return {
+		id: base.id,
+		name: base.name,
+		iconUrl: base.icon,
+		os: base.os,
+		hasParameters: false,
+	};
+}
+
+function detailsUrl(baseId: string): string {
+	return `https://registry.coder.com/templates/${baseId}`;
+}
+
+export const BaseInfraSelectStep: FC<BaseInfraSelectStepProps> = ({
+	selectedBaseId,
+	onSelectBase,
+}) => {
+	const { data, error, isLoading } = useQuery(templateBuilderBases());
+
+	if (isLoading) {
+		return <Loader />;
+	}
+
+	if (error) {
+		return <ErrorAlert error={error} />;
+	}
+
+	const bases = data?.bases ?? [];
+
+	return (
+		<div role="radiogroup" aria-label="Base infrastructure templates">
+			<h2 className="text-lg font-semibold mb-1">Pick a base template</h2>
+			<p className="text-sm text-content-secondary mb-4">
+				Select your infrastructure foundation.
+			</p>
+			<div className="grid grid-cols-1 sm:grid-cols-2 lg:grid-cols-3 gap-4">
+				{bases.map((base) => (
+					<TemplateCard
+						key={base.id}
+						name={base.name}
+						description={base.description}
+						iconUrl={base.icon}
+						detailsUrl={detailsUrl(base.id)}
+						selected={base.id === selectedBaseId}
+						onSelect={() => onSelectBase(toSelectedBaseMeta(base))}
+					/>
+				))}
+			</div>
+		</div>
+	);
+};

site/src/pages/TemplateBuilder/TemplateBuilderPageView.tsx

@@ -9,6 +9,7 @@ import {
 	PageHeaderTitle,
 } from "#/components/PageHeader/PageHeader";
 import { docs } from "#/utils/docs";
+import { BaseInfraSelectStep } from "./BaseInfraSelectStep";
 import { SelectionSummary } from "./SelectionSummary";
 import {
 	findNextVisibleIndex,
@@ -56,7 +57,7 @@ export const TemplateBuilderPageView: FC<TemplateBuilderPageViewProps> = ({
 	};
 
 	return (
-		<Margins>
+		<Margins className="pb-12">
 			<PageHeader>
 				<PageHeaderTitle>Create new template</PageHeaderTitle>
 				<PageHeaderSubtitle>
@@ -72,11 +73,18 @@ export const TemplateBuilderPageView: FC<TemplateBuilderPageViewProps> = ({
 			<div className="flex gap-8">
 				{/* Main content area */}
 				<div className="flex-1 min-w-0">
-					<div className="rounded-lg border border-solid border-border bg-surface-primary p-6 min-h-[400px]">
-						<p className="text-sm text-content-secondary">
-							Step: {currentStep.id}
-						</p>
-					</div>
+					{currentStep.id === "base-infra" ? (
+						<BaseInfraSelectStep
+							selectedBaseId={state.selectedBase?.id ?? null}
+							onSelectBase={(base) => dispatch({ type: "SET_BASE", base })}
+						/>
+					) : (
+						<div className="rounded-lg border border-solid border-border bg-surface-primary p-6 min-h-[400px]">
+							<p className="text-sm text-content-secondary">
+								Step: {currentStep.id}
+							</p>
+						</div>
+					)}
 
 					{/* Navigation controls */}
 					<div className="flex justify-end mt-6 gap-2">

site/src/pages/TemplateBuilder/wizardState.ts

@@ -8,7 +8,7 @@ import type {
  * UI-only metadata for the selected base template.
  * Kept separate from the API request payload.
  */
-type SelectedBaseMeta = {
+export type SelectedBaseMeta = {
 	id: string;
 	name: string;
 	iconUrl?: string;