refactor(button): consolidate button styles into Button.astro

Extends Button.astro with a center prop, native <button> element support
(type, disabled, aria-describedby), harmonised padding (.65rem), and
transition. Removes global .button, .button-small, .button-light, and
.button-center rules from components.css, layout.css, and typography.css.
Converts all inline class="button" usages across Header, HomeHero,
HomeFinancialHelp, HomeServicesIntro, FormShell, TeamPage, CareersPage,
and FaqPage to the <Button> component. Updates three scoped style
selectors to :global() to reach the child component's elements.

www/src/components/home/HomeServicesIntro.astro

@@ -1,4 +1,5 @@
 ---
+import Button from '../Button.astro';
 import type { ServicesIntroSection } from '../../types/home-sections';
 interface Props { section: ServicesIntroSection }
 const { section } = Astro.props;
@@ -31,7 +32,7 @@ const { section } = Astro.props;
       <div>
         <h2 class="no-balance">{section.commitmentsHeading}</h2>
         <ul class="source-list two-column-list">{section.commitments.map((item) => <li>{item}</li>)}</ul>
-        <a class="button" href={section.commitmentsCta.href}>{section.commitmentsCta.label}</a>
+        <Button href={section.commitmentsCta.href}>{section.commitmentsCta.label}</Button>
       </div>
       <img class="gear-photo gear-photo-right" src={`/assets/images/${section.commitmentsImage}`} alt={section.commitmentsImageAlt} />
     </div>