[web] Address design QA items on pricing page (#33682)

- pricing table: integration icons gap uses --spacing-06 (horizontal),
  integrations content gap uses --spacing-04 (vertical)
- Student card no longer renders with the green stroke highlight
- Interstitial H1 wrapped in .main-heading-section so its spacing
  matches the pricing page

Part of #33619.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
GitOrigin-RevId: 12ddd223f68c776c06a3d5dc5faa841819baae90

services/web/frontend/stylesheets/pages/plans-2026.scss

@@ -693,7 +693,7 @@
     align-items: center;
     display: flex;
     flex-wrap: wrap;
-    gap: var(--spacing-04);
+    gap: var(--spacing-06);
   }
 
   .plans-feature-table-section-icon-wrapper {
@@ -722,7 +722,7 @@
   .plans-feature-table-integrations-content {
     display: flex;
     flex-direction: column;
-    gap: var(--spacing-03);
+    gap: var(--spacing-04);
   }
 
   .plans-feature-table-integrations-label {