feat: hero profit cluster, dark text fix, footer, privacy/terms pages

- Replace hero right panel with liquid-glass profit cluster (กำไร + Marketing/AI/Business Knowledge)
- Fix dark-on-dark text visibility in all inverted sections (scene-dark, page-section)
- Add Footer component with liquid-glass design, contact info, legal links
- Add privacy and terms placeholder pages
- Update PageShell to include Footer on all pages

src/components/PageShell.astro

@@ -1,5 +1,6 @@
 ---
 import '../styles/global.css';
+import Footer from './Footer.astro';
 import { formEndpoint, problems, services } from '../data/site.js';
 
 const {
@@ -111,6 +112,8 @@ const {
       <slot />
     </main>
 
+    <Footer />
+
     <button class="floating-cta" type="button" data-floating-cta data-open-lead>ส่งโจทย์ให้เราดู</button>
     <div class="panel-backdrop" data-panel-backdrop data-close-lead></div>
     <aside class="lead-panel liquid-glass liquidGlass-wrapper" aria-labelledby="lead-title" aria-hidden="true" data-lead-panel data-endpoint={formEndpoint}>