Refactor frontend sections to use Tailwind CSS

Convert custom CSS styling to Tailwind utility classes across marketing
and about pages. Improve responsive layouts in feature grids, service
lists, and sliders. Consolidate section headers using the shared
SectionHeader component to maintain visual consistency.

apps/frontend/src/styles/theme.css

@@ -136,6 +136,8 @@
     /* Amber - CTA/強調 */
     --color-tarawera: #2d3748;
     /* Tarawera - 深色文字 */
+    --color-pale-purple: oklch(62.664% 0.15547 290.298);
+    /* Pale Purple - CTA/強調 */
     --color-nav-link: var(--color-gray-200);
     /* Navigation Link - 使用灰色系 */