fix(unity-bootstrap-theme): hero small CTA overflow at <768px (UDS-2171)

[juanmitriatti]

Summary

• Fixes the QA finding from UDS-2171: at viewports between mobile and the 768px breakpoint (e.g. 764×921, 200% zoom on small screens, mobile DevTools view), the Small Hero CTA button hangs out of the hero image into the section below.
• Root cause: `.btn-row` used `position: absolute; top: 15rem` (240px) inside a container with `min-height: 16rem` (256px) and default `overflow: visible` — button bottom (240+~44=284) exceeded the container by ~28-36px until `min-width: 768px` swapped to grid placement.
• Fix scoped to `uds-hero-sm` only: replace the absolute positioning with the same grid-flow pattern already used at tablet+ (`grid-row: 4; position: relative; margin-top: 1rem`). Container now expands naturally to contain the button. `uds-hero-md` and `uds-hero-lg` are intentionally untouched (not part of the QA report).

Diff

```diff
@@ div.uds-hero-sm.has-btn-row .btn-row @@
align-self: flex-start;
flex-wrap: wrap;

•  top: 15rem;
  

•  grid-row: 4;
  

•  position: relative;
  

•  margin-top: 1rem;
  

```

3 lines added, 1 removed. `yarn css-lint` passes; full SCSS compiles cleanly.

Test plan

Tested in Chrome at multiple viewports (computed styles + visual screenshots):

viewport	hero h	btn position	overflow
764×921 (the reported bug)	260px	rel, grid-row:4, mt:16	−48px (contained ✓)
767×921 (just below breakpoint)	260px	rel, grid-row:4	−48px ✓
500×921 (mobile)	256px	rel, grid-row:4	−48px ✓
800×921 (tablet+)	260px	rel, grid-row:4, mt:16	−48px ✓ (identical to existing tablet+ behavior)
764×921 Medium Hero	336px	abs, top:320 (untouched)	unchanged from main

• Verify on `websparkci-dev-asufactory1.acquia.asu.edu/hero` after deploy
• Confirm 200% zoom at small screens shows button contained
• Confirm Medium/Large heroes look unchanged (out of scope for this ticket)

[asu-jenkins-devops]

Storybook deployed at https://unity-uds-staging.s3.us-west-2.amazonaws.com/pr-1688/index.html