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.

public/sw.js

@@ -9,7 +9,7 @@
  *
  * Bumping CACHE_VERSION is how a new deploy invalidates old assets.
  */
-const CACHE_VERSION = "freezmo-v15";
+const CACHE_VERSION = "freezmo-v16";
 const SHELL_CACHE = `${CACHE_VERSION}-shell`;
 const ASSET_CACHE = `${CACHE_VERSION}-assets`;