Refactor hardcoded uses of rem unit

[mkrause]

There are still a lot of places where we use hardcoded rem units. This makes it harder to integrate Baklava into an app that has a different root font-size (e.g. apps using the font-size: 62.5% trick to get 1rem = 10px). Instead, we should convert these to use spacing tokens, or other suitable units that don't depend on the root styling.

$ grep -r rem src | grep scss | grep -v src/styling | grep -v src/docs | grep -v rem-from-px
src/components/tables/MultiSearch/MultiSearch.scss:    border: 0.2rem solid $neutral-color-1;
src/components/tables/MultiSearch/MultiSearch.scss:      padding: 1rem;
src/components/tables/DataTable/table/DataTable.module.scss:          padding-block-end: 1rem;
src/components/tables/DataTable/table/DataTable.module.scss:        padding: 0.6rem 0;
src/components/tables/DataTable/table/DataTable.module.scss:          padding: 0.6rem 0;
src/components/tables/DataTable/table/DataTable.module.scss:          margin: 1.5rem 0;
src/components/tables/DataTable/table/DataTable.module.scss:          padding: 1.5rem;
src/components/tables/DataTable/table/DataTablePlaceholder.module.scss:      gap: 1rem;
src/components/forms/controls/Select/Select.module.scss:  $bk-select-min-inline-size: 14rem;
src/components/forms/controls/Select/Select.module.scss:    max-block-size: 18rem;
src/components/forms/controls/Select/Select.module.scss:      translate: 0 -1rem;
src/components/forms/controls/Select/Select.module.scss:        translate: 0 -1rem;
src/components/forms/fields/TextAreaField/TextAreaField.module.scss:      --empty: ; // Prevent empty class from being removed
src/components/forms/fields/InputField/InputField.module.scss:      --empty: ; // Prevent empty class from being removed
src/components/util/overlays/modal/ModalProvider.module.scss:    max-inline-size: 30rem;
src/components/overlays/ToastProvider/ToastProvider.module.scss:      margin: 1rem;
src/components/overlays/Tooltip/Tooltip.module.scss:    max-inline-size: 30rem;
src/components/overlays/Tooltip/Tooltip.module.scss:      translate: 0 0.3rem;
src/components/overlays/Tooltip/Tooltip.module.scss:        translate: 0 0.3rem;
src/components/overlays/DropdownMenu/DropdownMenu.module.scss:  $bk-dropdown-menu-min-width: 14rem;
src/components/overlays/DropdownMenu/DropdownMenu.module.scss:    max-block-size: 18rem;
src/components/overlays/DropdownMenu/DropdownMenu.module.scss:        translate: 0 -1rem;
src/components/overlays/DropdownMenu/DropdownMenu.module.scss:            translate: 0 -1rem;
src/components/overlays/DialogModal/DialogModal.module.scss:// Note: `$blur` should be in px, not rem (blur effect should be independent of any font size scaling)
src/components/overlays/DialogModal/DialogModal.module.scss:      inset: 1rem;
src/components/actions/Button/Button.module.scss:    // Experiment: implementing `unstyled` using `revert-layer` rather than by removing the class names
src/components/containers/Disclosure/Disclosure.module.scss:        //block-size: 20rem; // This works
src/components/containers/Disclosure/Disclosure.module.scss:        --summary-block-size: calc(var(--bk-disclosure-padding-block) + 1.6rem);
src/components/containers/Panel/Panel.module.scss:  min-block-size: 4rem;
src/components/containers/Banner/Banner.module.scss:    --bk-banner-indent: calc(#{bk.$spacing-3} + 1.2rem + #{bk.$spacing-2}); // Spacing + icon width + spacing
src/components/graphics/PlaceholderEmpty/PlaceholderEmpty.module.scss:        --bk-icon-size: 6rem;
src/components/graphics/PlaceholderEmpty/PlaceholderEmpty.module.scss:        max-block-size: 5rem;
src/components/graphics/PlaceholderEmpty/PlaceholderEmpty.module.scss:        --bk-icon-size: 4rem;
src/components/graphics/PlaceholderEmpty/PlaceholderEmpty.module.scss:        max-block-size: 3rem;
src/layouts/AppLayout/AppLayout.module.scss:      font-size: 0.8rem;
src/layouts/AppLayout/Header/UserMenu.module.scss:    min-inline-size: 1rem; // Allow it to shrink down to just the icon on small screens
src/layouts/AppLayout/Header/AccountSelector.module.scss:    min-inline-size: 3rem;
src/layouts/AppLayout/Header/AccountSelector.module.scss:      margin-inline-start: -0.2rem;
src/layouts/AppLayout/Header/AccountSelector.module.scss:      margin-block-end: -0.2rem;
src/layouts/AppLayout/Header/SysadminSwitcher.module.scss:    --icon-size: 1.5rem;
src/layouts/AppLayout/Header/SysadminSwitcher.module.scss:      padding-inline-start: 0.1rem;
src/layouts/AppLayout/Header/SysadminSwitcher.module.scss:      padding-inline-start: 0.1rem;
src/layouts/AppLayout/Header/SolutionSelector.module.scss:    min-inline-size: 3rem;
src/layouts/AppLayout/Header/SolutionSelector.module.scss:      margin-inline-start: -0.2rem;
src/layouts/AppLayout/Header/SolutionSelector.module.scss:      margin-block-end: -0.2rem;
src/typography/Prose/Prose.mixins.scss:  font-size: 1rem;