style: Tighten section spacing and fix GitHub logo padding

- Reduce section header top margin from 3rem to var(--space-6) for
  better density alignment with Precision & Density design direction
- Fix GitHub logo padding to 4px grid (0 2px → 0 4px)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: MaxDaten

src/templates/template.css

@@ -209,7 +209,7 @@ h2 {
     font-weight: 600;
     line-height: 1.3;
     letter-spacing: -0.01em;
-    margin: 3rem 0 var(--space-5);
+    margin: var(--space-6) 0 var(--space-5);
     padding-bottom: var(--space-3);
     color: var(--text-heading);
     position: relative;
@@ -433,7 +433,7 @@ code {
    GitHub Logo
    -------------------------------------------------------------------------- */
 #github-logo {
-    padding: 0 2px;
+    padding: 0 4px;
     vertical-align: text-bottom;
 }