fix(site): enable sticky positioning inside #main-content (#26907)

ref DEVEX-567

Fixes a bug I noticed where #26881 claims to have made the template
builder's `SelectionSummary` sidebar sticky-positioned, but the sidebar
position wasn't actually sticky in practice:

## template builder's `SelectionSummary` (top right)

### before


https://github.com/user-attachments/assets/1e9360b5-4b0e-45b4-a859-f8edab51add7

### after


https://github.com/user-attachments/assets/3cee75e9-9813-401e-9922-d2563185f8f5

---

Why I removed `overflow-y-auto` from `#main-content`:

>`position: sticky` resolves against the nearest ancestor scroll
container — any ancestor whose overflow is not visible. `#main-content`
had `overflow-y-auto`, so it _was_ that container for the sidebar. But
because the layout wrapper is `min-h-screen`, `<main>` grows to fit its
content and never actually scrolls — the window scrolls. So the sidebar
was pinning relative to a container that never moves → no effect.

That was necessary for the main goal of this PR, which was to fix sticky
positioning for the template builder's `SelectionSummary` sidebar.
However, removing that style from `#main-content` affected 2 other
sticky-positioned elements within the site:

>Two other pages have sticky elements that, like our sidebar, were
resolving against the non-scrolling `<main>` and were therefore
effectively inert:
>- CreateWorkspacePageView.tsx:397 — `sticky top-5` side panel
>- modules/templates/TemplateFiles/TemplateFiles.tsx:64 — `sticky top-8`
file tree

^Like `SelectionSummary`, these 2 elements hadn't been behaving with
sticky positioning as expected; they would just scroll away past the top
of the screen.

For all 3 of these sticky elements, since their `top` is now relative to
the window instead of `#main-content`, they have to be positioned
farther downward so that they don't get covered by the navbar.

## `TemplateFiles`' `TemplateFileTree` (top left)

### before


https://github.com/user-attachments/assets/ee79ea36-1a47-4b1b-86ef-c65b493ea455

### after


https://github.com/user-attachments/assets/a1b0a949-b5e3-4434-acaa-ab24e82b1669

## `CreateWorkspacePageView`'s "Go back" button (top left)

### before


https://github.com/user-attachments/assets/00b076f6-8e55-4a96-8b92-d33d92c85334

### after


https://github.com/user-attachments/assets/10ae05e1-3112-4dfc-84b4-2c44fee23e06

co-authored with Claude Code

---------

Co-authored-by: Jeremy Ruppel <jeremyruppel@users.noreply.github.com>

site/src/components/Form/Form.tsx

@@ -99,7 +99,7 @@ export const FormSection: FC<FormSectionProps> = ({
 		>
 			<div
 				className={cn(
-					"w-full shrink-0 top-6",
+					"w-full shrink-0 top-24",
 					direction === "horizontal" && "lg:sticky lg:max-w-[312px]",
 					classes.sectionInfo,
 				)}

site/src/modules/dashboard/DashboardLayout.tsx

@@ -43,7 +43,7 @@ export const DashboardLayout: FC = () => {
 					id="main-content"
 					tabIndex={-1}
 					className={cn(
-						"relative flex flex-col flex-1 min-h-0 overflow-y-auto",
+						"relative flex flex-col flex-1 min-h-0",
 						"focus:outline-none",
 					)}
 				>

site/src/modules/templates/TemplateFiles/TemplateFiles.tsx

@@ -61,7 +61,8 @@ export const TemplateFiles: FC<TemplateFilesProps> = ({
 	return (
 		<div>
 			<div className="flex items-start gap-8">
-				<div className="sticky top-8 w-[240px] shrink-0 overflow-auto rounded-lg border border-solid border-surface-quaternary py-1">
+				{/* top-28 (112px) is a sum of nav height (72px) and TemplateLayout's LinkTabs' mb-10 (40px) */}
+				<div className="sticky top-28 w-[240px] shrink-0 overflow-auto rounded-lg border border-solid border-surface-quaternary py-1">
 					<TemplateFileTree
 						fileTree={fileTree}
 						onSelect={(path: string) => {

site/src/pages/CreateTemplateGalleryPage/StarterTemplates.tsx

@@ -65,7 +65,7 @@ export const StarterTemplates: FC<StarterTemplatesProps> = ({
 	return (
 		<div className="flex flex-row gap-8 items-start">
 			{starterTemplatesByTag && tags && (
-				<div className="flex flex-col gap-4 w-[202px] shrink-0 sticky">
+				<div className="flex flex-col gap-4 w-[202px] shrink-0 sticky top-[88px]">
 					<h2 className="m-0 text-base font-normal text-content-primary">
 						Choose a starter template
 					</h2>

site/src/pages/CreateWorkspacePage/CreateWorkspacePageView.tsx

@@ -398,7 +398,7 @@ export const CreateWorkspacePageView: FC<CreateWorkspacePageViewProps> = ({
 				<button
 					onClick={onCancel}
 					type="button"
-					className="flex items-center gap-2 bg-transparent border-none text-content-secondary hover:text-content-primary translate-y-12"
+					className="flex items-center gap-2 bg-transparent border-none text-content-secondary hover:text-content-primary translate-y-[68px]"
 				>
 					<ArrowLeftIcon size={20} />
 					Go back

site/src/pages/TemplateBuilder/TemplateBuilderPageView.tsx

@@ -182,8 +182,8 @@ export const TemplateBuilderPageView: FC<TemplateBuilderPageViewProps> = ({
 					{currentStep.id === "base-infra" && <TemplateAlternatives />}
 				</div>
 
-				{/* Sidebar */}
-				<div className="w-64 shrink-0 hidden md:block sticky top-0 self-start">
+				{/* Sidebar (top position is 72px so that it can sit below nav) */}
+				<div className="w-64 shrink-0 hidden md:block sticky top-[72px] self-start">
 					<SelectionSummary
 						currentStep={currentStep.group}
 						selectedTemplate={