Button component: add `dark` variant (ex Bootstrap `.btn-dark`, solid #212529).

- turbo: 3 `alert alert-secondary` info boxes -> `rounded-md border bg-surface
  p-4`; `btn btn-dark` submit -> `<twig:Button variant="dark">`
- translator: 6 `form-control` + 1 `form-select` -> `w-full bg-body border
  rounded-md px-3 py-1.5`; `.small` labels -> `text-sm`
- native: `fw-bold` -> `font-bold`; `text-body-secondary` -> `text-muted`
- live_component: `btn btn-md btn-outline-primary` doc link -> `<twig:Button>`

Note: turbo's form_row `form-floating` is left for the form-theme migration.
Verified in light/dark mode across all four pages.

The GitHub/copy icon buttons in CodeBlockButtons and CodeBlockInline drop their
Bootstrap `.btn .btn-link .btn-sm` classes. Their styling now lives in
_Terminal.css scoped to `.code-buttons`, the common wrapper of both components.

- Style `.code-buttons :is(a, button)` directly (color n-200 -> n-100 on hover,
  4px 8px padding, transparent bg) instead of relying on the Bootstrap button
  base + the `.Terminal_actions .btn-link/.btn-copy` overrides.
- Keep `.btn-copy` as a CSS hook with `position: relative` (offset parent for
  the `.copied button::after` copy-feedback ring), so it no longer depends on
  vendor/_bootstrap.scss.
- `data-bs-toggle="tooltip"` left untouched (Bootstrap JS removal is Phase 10).

Verified the buttons render identically (color, padding, position) in code
block headers, light and dark.

base.html.twig and _header.html.twig were already migrated in Phase 7,
so the footer is all that remained for the global layout.

- Drop the leftover `--bs-gutter-x: 0` inline style (Bootstrap row var,
  redundant with the Tailwind `grid gap-x-6`)
- Convert inline font-size / text-transform / font-weight / gap styles to
  Tailwind utilities (text-[11px], text-xs, text-2xl, uppercase, font-bold,
  gap-2)
- Remove the now-empty wrapper <span> around the Upsun link

Layout and rendering verified identical in light and dark mode.