feat(odin): use the Odin mark as the sidebar nav icon

Replace the chat-bubble glyph on the "Odin" nav item with the OdinMark, and
make the mark's SVG sizing !important so the nav's `& svg { width:18 }` rule
can't shrink it (its `size` prop stays authoritative everywhere).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/admin/ui/navData.tsx

@@ -1,4 +1,5 @@
 import { type ReactNode } from "react";
+import OdinMark from "../components/odin/OdinMark";
 
 export interface MenuItem {
   path: string;
@@ -43,16 +44,7 @@ export const menuSections: MenuSection[] = [
         label: "Odin",
         permission: "ai.use",
         end: true,
-        icon: (
-          <svg
-            viewBox="0 0 24 24"
-            fill="none"
-            stroke="currentColor"
-            strokeWidth="2"
-          >
-            <path d="M21 11.5a8.38 8.38 0 0 1-.9 3.8 8.5 8.5 0 0 1-7.6 4.7 8.38 8.38 0 0 1-3.8-.9L3 21l1.9-5.7a8.38 8.38 0 0 1-.9-3.8 8.5 8.5 0 0 1 4.7-7.6 8.38 8.38 0 0 1 3.8-.9h.5a8.48 8.48 0 0 1 8 8v.5z" />
-          </svg>
-        ),
+        icon: <OdinMark size={22} />,
       },
     ],
   },