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/components/odin/OdinMark.tsx

@@ -29,8 +29,10 @@ const glow = keyframes`
 const Glyph = styled("svg", {
   shouldForwardProp: (p) => p !== "thinking" && p !== "reduce",
 })<{ thinking: boolean; reduce: boolean }>(({ thinking, reduce }) => ({
-  width: "70%",
-  height: "70%",
+  // !important so an ancestor `& svg { width }` rule (e.g. SidebarNav forces
+  // 18px) can't override the mark's own sizing — `size` stays authoritative.
+  width: "70% !important",
+  height: "70% !important",
   overflow: "visible",
   display: "block",
   ...(reduce