fix: add scroll-to-top button to remaining pages (closes #920)

[Quantumboy80]

Closes #920

What was actually going on

While investigating, I found the feature was already half-built:

• script.js already has a working scrollToTop() function and a scroll
  listener wired to #scrollTopBtn
• style.css already had a #scrollTopBtn:hover rule, but no base rule
  (no position, size, or default display: none) — so the button was
  unstyled wherever it existed
• The button element itself was present on index.html, about.html,
  privacy-policy.html, refund-policy.html, and terms-of-service.html,
  but missing from birthday.html, contact.html, favourites.html,
  and products.html

Changes

• Added the missing base CSS for #scrollTopBtn (fixed position,
  bottom-right, circular, dark mode variant, mobile sizing)
• Added the missing <button id="scrollTopBtn" onclick="scrollToTop()">
  element to the 4 pages that didn't have it
• Positioned per the existing comment in the WhatsApp button's CSS
  (bottom: 80px; /* sits above the back-to-top button */), so the two
  buttons now stack correctly instead of overlapping

Testing

Tested locally on all 4 fixed pages plus the 2 pages that already had
the button (to confirm no regression), in both light/dark mode and at
mobile width:

• Button appears after ~300px scroll, hidden at top
• Smooth-scrolls to top on click
• Sits correctly below the WhatsApp button, no overlap
• Responsive sizing on mobile

Note for maintainer

Not part of this PR, but I noticed scrollToTop() is defined twice in
script.js (once ~line 1020, once ~line 1621) — harmless dead code,
happy to clean it up in a separate PR if useful.

[vercel]

@Quantumboy80 is attempting to deploy a commit to the adithyansubramani1-1657's projects Team on Vercel.

A member of the Team first needs to authorize it.