Update Nexumi banner assets and adjust SVG positioning for improved visibility

- Replaced the existing PNG banner with an updated version for better quality.
- Adjusted the SVG positioning of the 'N' mark and wordmark to prevent overlap with the Discord avatar, enhancing overall design clarity.

docs/logo/nexumi-banner.svg

@@ -36,8 +36,8 @@
     <path d="M900 100 L1120 160 L1040 320"/>
   </g>
 
-  <!-- N mark (scaled from logo proportions) -->
-  <g transform="translate(110,40) scale(1.05)">
+  <!-- N mark — shifted up so Discord avatar does not cover it -->
+  <g transform="translate(90,-70) scale(0.92)">
     <g fill="none" stroke="url(#mark)" stroke-linecap="round" stroke-linejoin="round">
       <path d="M170 362 V150 L342 362 V150" stroke-width="78" stroke-opacity="0.10"/>
       <path d="M170 362 V150 L342 362 V150" stroke-width="52" stroke-opacity="0.16"/>
@@ -53,6 +53,6 @@
   </g>
 
   <!-- wordmark -->
-  <text x="620" y="250" fill="#E6E6EA" font-family="Inter, ui-sans-serif, system-ui, sans-serif" font-size="92" font-weight="700" letter-spacing="-2">Nexumi</text>
-  <text x="624" y="300" fill="#9A9AA5" font-family="Inter, ui-sans-serif, system-ui, sans-serif" font-size="28" font-weight="500">nexumi.de</text>
+  <text x="560" y="200" fill="#E6E6EA" font-family="Inter, ui-sans-serif, system-ui, sans-serif" font-size="92" font-weight="700" letter-spacing="-2">Nexumi</text>
+  <text x="564" y="250" fill="#9A9AA5" font-family="Inter, ui-sans-serif, system-ui, sans-serif" font-size="28" font-weight="500">nexumi.de</text>
 </svg>