feat(site/src/pages/TemplateBuilder): add scroll overflow and sticky sidebar to wizard steps (#26881)

Add vertical overflow scrolling to the TemplateBuilder wizard step
content areas and make the selection sidebar sticky.

## Changes

- **Scroll on inner content**: Each step's inner content area (below the
heading and any tabs/search) gets `max-h-[calc(100vh-Npx)]
overflow-y-auto`, keeping headings and tab bars visible while the
content scrolls.
- Base template select and module select grids: 420px offset (accounts
for navbar, page header, card padding, tab bar, and nav controls)
  - Base template parameters and module settings: 340px offset
  - Customizations step: no scroll, flows naturally
- **Sticky sidebar**: The selection summary sidebar uses `sticky top-0
self-start` to stay visible while scrolling.
- **Scroll reset on navigation**: `window.scrollTo(0, 0)` in
Back/Continue handlers so users start at the top of each step.

> 🤖 Generated by Coder Agents

<details><summary>Implementation plan</summary>

The approach uses `max-h` with `overflow-y-auto` on the inner content
divs of each step component, placed below the step heading and any
tabs/search controls. This keeps the heading fixed while the content
scrolls. The offset values account for the vertical space consumed by
the navbar, page header, card border/padding, and navigation controls.

The `DashboardFullPage` flex-fill approach was explored but abandoned
because the viewport-constrained card was too short to show content on
smaller screens. The standard `Margins` + `pb-12` layout with per-step
`max-h` provides a better balance.
</details>

site/src/pages/TemplateBuilder/BaseInfraSelectStep.tsx

@@ -55,7 +55,8 @@ export const BaseInfraSelectStep: FC<BaseInfraSelectStepProps> = ({
 				Select your infrastructure foundation.
 			</TemplateBuilderSubtitle>
 
-			<div className="grid grid-cols-1 sm:grid-cols-2 lg:grid-cols-3 gap-4">
+			{/* 420px accounts for navbar, page header, card padding, tab bar, and nav controls */}
+			<div className="grid grid-cols-1 sm:grid-cols-2 lg:grid-cols-3 gap-4 max-h-[calc(100vh-420px)] overflow-y-auto">
 				{bases.map((base) => (
 					<TemplateCard
 						key={base.id}

site/src/pages/TemplateBuilder/BaseTemplateParametersStep.tsx

@@ -119,29 +119,32 @@ export const BaseTemplateParametersStep: FC<
 				Your base template requires customizations.
 			</TemplateBuilderSubtitle>
 
-			<TemplateConfiguration
-				name={base?.name ?? "Base Template"}
-				description={base?.description ?? ""}
-				iconUrl={base?.icon}
-				detailsUrl={detailsUrl(baseId)}
-				fields={fields}
-			>
-				{prerequisites && (
-					<div className="mt-6">
-						<MemoizedMarkdown
-							className={cn(
-								"text-sm font-normal",
-								"[&_h2]:mt-6 [&_h2]:text-base [&_h2]:font-semibold",
-								"[&_h3]:mt-2 [&_h3]:mb-1 [&_h3]:text-sm [&_h3]:font-semibold",
-								"[&_p]:mb-3 [&_p]:text-content-secondary",
-								"[&_a]:font-normal",
-							)}
-						>
-							{prerequisites}
-						</MemoizedMarkdown>
-					</div>
-				)}
-			</TemplateConfiguration>
+			{/* 340px accounts for navbar, page header, card padding, and nav controls */}
+			<div className="max-h-[calc(100vh-340px)] overflow-y-auto">
+				<TemplateConfiguration
+					name={base?.name ?? "Base Template"}
+					description={base?.description ?? ""}
+					iconUrl={base?.icon}
+					detailsUrl={detailsUrl(baseId)}
+					fields={fields}
+				>
+					{prerequisites && (
+						<div className="mt-6">
+							<MemoizedMarkdown
+								className={cn(
+									"text-sm font-normal",
+									"[&_h2]:mt-6 [&_h2]:text-base [&_h2]:font-semibold",
+									"[&_h3]:mt-2 [&_h3]:mb-1 [&_h3]:text-sm [&_h3]:font-semibold",
+									"[&_p]:mb-3 [&_p]:text-content-secondary",
+									"[&_a]:font-normal",
+								)}
+							>
+								{prerequisites}
+							</MemoizedMarkdown>
+						</div>
+					)}
+				</TemplateConfiguration>
+			</div>
 		</>
 	);
 };

site/src/pages/TemplateBuilder/ModuleSelectStep.tsx

@@ -243,7 +243,8 @@ export const ModuleSelectStep: FC<ModuleSelectStepProps> = ({
 				</TabsList>
 			</Tabs>
 
-			<div className="grid grid-cols-1 sm:grid-cols-2 lg:grid-cols-3 gap-4">
+			{/* 420px accounts for navbar, page header, card padding, search, tabs, and nav controls */}
+			<div className="grid grid-cols-1 sm:grid-cols-2 lg:grid-cols-3 gap-4 max-h-[calc(100vh-420px)] overflow-y-auto">
 				{visibleModules.length ? (
 					visibleModules.map((m) => (
 						<ModuleCard

site/src/pages/TemplateBuilder/ModuleSettingsStep.tsx

@@ -127,7 +127,8 @@ export const ModuleSettingsStep: FC<ModuleSettingsStepProps> = ({
 				Set values for module variables.
 			</TemplateBuilderSubtitle>
 
-			<div className="flex flex-col gap-6">
+			{/* 340px accounts for navbar, page header, card padding, and nav controls */}
+			<div className="flex flex-col gap-6 max-h-[calc(100vh-340px)] overflow-y-auto">
 				{selectedModules.map((mod) => {
 					const configurableVars = mod.variables.filter((v) => !v.sensitive);
 					const sensitiveVars = mod.variables.filter((v) => v.sensitive);

site/src/pages/TemplateBuilder/TemplateBuilderPageView.tsx

@@ -1,4 +1,5 @@
 import { type FC, type ReactNode, useReducer, useState } from "react";
+
 import { useQuery } from "react-query";
 import { templateBuilderModules } from "#/api/queries/templateBuilder";
 import type {
@@ -14,6 +15,7 @@ import {
 	PageHeaderSubtitle,
 	PageHeaderTitle,
 } from "#/components/PageHeader/PageHeader";
+
 import { docs } from "#/utils/docs";
 import { BaseInfraSelectStep } from "./BaseInfraSelectStep";
 import {
@@ -68,6 +70,7 @@ export const TemplateBuilderPageView: FC<TemplateBuilderPageViewProps> = ({
 
 	const currentIndex = nearestVisible(stepIndex, state);
 	const currentStep = WIZARD_STEPS[currentIndex];
+
 	const nextIndex = findNextVisibleIndex(currentIndex, state);
 	const prevIndex = findPrevVisibleIndex(currentIndex, state);
 	const isFirstStep = prevIndex === -1;
@@ -82,6 +85,7 @@ export const TemplateBuilderPageView: FC<TemplateBuilderPageViewProps> = ({
 	);
 
 	const handleBack = () => {
+		window.scrollTo(0, 0);
 		setStepIndex(prevIndex);
 	};
 
@@ -90,6 +94,7 @@ export const TemplateBuilderPageView: FC<TemplateBuilderPageViewProps> = ({
 			onCreateTemplate(state);
 			return;
 		}
+		window.scrollTo(0, 0);
 		setStepIndex(nextIndex);
 	};
 
@@ -158,7 +163,7 @@ export const TemplateBuilderPageView: FC<TemplateBuilderPageViewProps> = ({
 				</div>
 
 				{/* Sidebar */}
-				<div className="w-64 shrink-0 hidden md:block">
+				<div className="w-64 shrink-0 hidden md:block sticky top-0 self-start">
 					<SelectionSummary
 						currentStep={currentStep.group}
 						selectedTemplate={