bootstrap: home carousel jQuery plugin -> react-bootstrap <Carousel>

Second (last) jQuery/BS4 JS blocker to the BS5 CSS swap. Replaces the
`bootstrap-carousel-swipe` jQuery plugin + `$(...).carousel()` init with
react-bootstrap's <Carousel> (native swipe, no jQuery) for both home carousels.

- Preserves the progressive `.lazy` background mechanism (blur -> full image on
  slide-in): the old `slide.bs.carousel` handler becomes <Carousel onSlide>,
  which marks the incoming index in component state.
- Keeps all per-slide class hooks (carousel-item-N, carousel-snd-item-N).
- CSS ports in Index-custom.scss for react-bootstrap's BS5 markup: indicators
  render as <button> (not <li>), and prev/next labels use .visually-hidden
  (BS4 had .sr-only) -> add a shim so labels stay hidden on current BS4 CSS.
- Drops the now-unused bootstrap-carousel-swipe dependency.

Full-app build boots clean. Needs a visual staging check of the home page.

package.json

@@ -14,7 +14,6 @@
     "@turf/buffer": "^5.1.5",
     "babel-runtime": "^6.26.0",
     "bcrypt": "^5.1.1",
-    "bootstrap-carousel-swipe": "0.0.6",
     "commonmark": "^0.28.1",
     "core-js": "^2.5.1",
     "crypto-random-hex": "^1.0.0",