feat(licenses): sidebar footer link to /sdk/licenses

Adds a discreet "Quellen & Lizenzen" link to the SDK sidebar footer
(below the existing Export button) pointing to the /sdk/licenses page
shipped in commit dfac940.

Part of Task #24 (AGB/Impressum audit) — the legal mandate that
attribution be discoverable for every output is now satisfied at
three layers:
- platform-wide overview reachable from every SDK page (this commit)
- per-export footer in compliance PDFs (commit 07cc00d)
- inline source badge per control via <SourceBadge> (commit dfac940)

admin-compliance/components/sdk/Sidebar/SDKSidebar.tsx

@@ -224,6 +224,19 @@ export function SDKSidebar({ collapsed = false, onCollapsedChange }: SDKSidebarP
             <span>Exportieren</span>
           </button>
         )}
+
+        {!collapsed && (
+          <a
+            href="/sdk/licenses"
+            className="mt-2 w-full flex items-center justify-center gap-2 px-4 py-2 text-xs text-gray-500 hover:text-gray-700 hover:bg-gray-100 rounded-lg transition-colors"
+            title="Quellen und Lizenzen aller verwendeten Compliance-Controls"
+          >
+            <svg className="w-3.5 h-3.5" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+              <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M9 12h6m-6 4h6m2 5H7a2 2 0 01-2-2V5a2 2 0 012-2h5.586a1 1 0 01.707.293l5.414 5.414a1 1 0 01.293.707V19a2 2 0 01-2 2z" />
+            </svg>
+            <span>Quellen &amp; Lizenzen</span>
+          </a>
+        )}
       </div>
     </aside>
   )