Add btn-theme default style, gray active item, menu shadow, and dark-mode btn-dark border

[MikeAlhayek]

Summary

Make the default look match a stock Bootstrap 5.3 select in light and dark modes.

• btn-theme style (new, now the default style). Renders light in light mode and dark in dark mode by following Bootstrap's CSS variables. This is the themed behavior the default btn-light had before.
• btn-light is stock again. It stays light in dark mode. Use btn-theme (or omit style) to follow the color mode.
• btn-dark border in dark mode. The toggle previously blended into the page because its border matched its background; it now uses --bs-border-color, like a native dark-mode input.
• Gray active item. The selected item is highlighted with --bs-secondary-bg (hover --bs-tertiary-bg) instead of the primary blue. Override --bs-dropdown-link-active-bg / --bs-dropdown-link-active-color on .bootstrap-select .dropdown-menu to restore the primary highlight.
• Menu drop shadow. The open menu uses --bs-box-shadow.

The Select all / Deselect all buttons follow btn-theme as well.

Breaking-ish

Anyone relying on data-style="btn-light" to follow dark mode should switch to btn-theme (or drop data-style). Default users see no change except the gray highlight and shadow.

Test plan

• New tests/e2e/theme-styles.spec.js covers the default style, btn-theme in both modes, btn-light in dark mode, btn-dark border, shadow and the gray active item.
• Existing e2e suite passes (31 tests, run against installed Chrome).
• npm run lint clean.
• Visually checked light and dark.

Docs (options.mdx, examples.mdx) and the changelog are updated.

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