Add logo, integrations section, improve spacing

- Real logo SVG in header and footer
- New Integrations section with ERP teaser (Odoo, SAP, DATEV, etc.)
- Notification channels (Email, WhatsApp, Telegram, Webhook, API, Slack)
- Improved grid spacing in features

public/logo.svg

@@ -0,0 +1,24 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 120" width="40" height="48">
+  <!-- Shield outline -->
+  <path d="M50 5 L95 25 L95 60 C95 85 75 105 50 115 C25 105 5 85 5 60 L5 25 Z" 
+        fill="#1A365D" 
+        stroke="#1A365D" 
+        stroke-width="4"
+        stroke-linejoin="round"/>
+  
+  <!-- Bell -->
+  <g transform="translate(50, 62)">
+    <!-- Bell top knob -->
+    <circle cx="0" cy="-28" r="5" fill="#E53E3E"/>
+    
+    <!-- Bell body -->
+    <path d="M-18 0 C-18 -18 -13 -23 0 -23 C13 -23 18 -18 18 0 L-18 0 Z" 
+          fill="white"/>
+    
+    <!-- Bell rim -->
+    <ellipse cx="0" cy="2" rx="22" ry="5" fill="white"/>
+    
+    <!-- Bell clapper -->
+    <ellipse cx="0" cy="11" rx="5" ry="4" fill="#E53E3E"/>
+  </g>
+</svg>