fix: Zielgruppe in Problem-Section, Mobile Feature-Cards

- Untertitel in Problem-Section: Zielgruppe definiert
  (Sicherheitsbehoerden, Redaktionen, Unternehmen)
- Feature-Cards auf Mobile volle Breite (Flex-Override)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vorschau/css/style.css

@@ -94,6 +94,7 @@ a { color:inherit; text-decoration:none; }
 
 .section-title { font-size:2rem; font-weight:700; color:var(--navy); text-align:center; margin-bottom:16px; letter-spacing:-0.01em; }
 .section-dark .section-title { color:var(--white); }
+.section-subtitle-light { color:rgba(255,255,255,0.6); }
 .section-subtitle { font-size:1.05rem; color:var(--text-light); text-align:center; max-width:600px; margin:0 auto 48px; }
 
 /* ==================== SECTION DIVIDERS ==================== */