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.

components/UploadScreen.tsx

@@ -719,9 +719,9 @@ export function UploadScreen() {
           </div>
         </section>
 
-        <div className="sticky bottom-[calc(env(safe-area-inset-bottom,0px)+4.6rem)] z-20 -mx-4 space-y-2 border-t border-[var(--hairline)] bg-white/65 px-4 py-3 backdrop-blur-2xl dark:bg-[color-mix(in_oklab,var(--surface)_70%,transparent)] sm:bottom-0 sm:mx-0 sm:rounded-[1.35rem] sm:border sm:px-5">
-          {/* Visually identical to the homework flow's CTA — same icon, shape
-              and helper line — only the label differs. */}
+        {/* Same CTA block as the homework flow: full-width button with the
+            helper line centred underneath, in the page flow — no sticky bar. */}
+        <div className="space-y-2">
           <Button
             size="lg"
             className="touch-target w-full"