feat(bet-form): add sticky action bar on BetForm while scrolling

Adds a sticky bottom action bar to BetForm that stays pinned to the
bottom of the form while the preset chips and amount input scroll
within a constrained container.

- Wraps QuickBetPresets and the amount input in a max-h overflow-y-auto
  scrollable container so the form stays compact on small viewports.
- The submit button (Place Bet) is now wrapped in a sticky bottom-0
  action bar that remains reachable while the content scrolls.
- Grows a top border + shadow once the body has been scrolled (tracked
  via onScroll), so it reads as a floating bar over content rather than
  empty space when everything fits without scrolling.
- Respects reducedMotion (suppresses the shadow transition).
- Adds focused unit tests for the sticky action bar behavior.

Closes #808

Author: waterWang