Drop the sticky bar: the Freeze my deck CTA now sits in the flow like the homework one

The last visible difference between the two CTAs was the container: the
deck button lived in a full-bleed sticky strip (translucent background,
hairline top border, its own padding), which also tinted the disabled chip
differently. The homework button sits plainly in the page flow.

- The CTA block is now a plain space-y-2 stack at the end of the form:
  full-width button + centred helper line, exactly like /homework.
- /create's page padding switches from the sticky-era .pb-cta (9rem, phone
  override 4.6rem) to .pb-tabbar, and lg:pb-20 to lg:pb-16 — identical to
  the homework page. The now-unused .pb-cta rules are gone.
- Result: same button width (358px @390 / 688px @1280), height, radius,
  helper placement and trailing space on both pages; neither has a sticky
  ancestor any more.

Service worker: freezmo-v16.

Verified: CTA-flow suite 26/26 (below-the-fold at rest, no sticky ancestor,
full-width, helper centred underneath, width/height/radius/trailing-space
parity at 390px and 1280px), desktop regression 74/74, mobile 51/51,
i18n 27/27. Lint + build clean.

app/create/page.tsx

@@ -28,7 +28,7 @@ export default async function CreatePage({
         <MobileSidebar />
         <main className="flex min-h-dvh min-w-0 flex-1 flex-col">
           <AppHeader />
-          <div className="mx-auto w-full max-w-3xl flex-1 px-4 pb-cta lg:px-10 lg:pb-20">
+          <div className="mx-auto w-full max-w-3xl flex-1 px-4 pb-tabbar lg:px-10 lg:pb-16">
             <UploadScreen />
           </div>
           <TabBar />