feat(ai): move the assistant to its own "Odin" sidebar page

Promote the AI chat from a dashboard widget to a dedicated page:
- new sidebar item "Odin" under the Přehled section, gated on ai.use, → /odin
- new /odin route + Odin page (reuses the assistant component, with a light
  ai.use guard)
- remove the widget from the dashboard
- rebrand the assistant header "Asistent" → "Odin" and give the thread more
  height now that it owns the page

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

src/admin/pages/Odin.tsx

@@ -0,0 +1,27 @@
+import Box from "@mui/material/Box";
+import Typography from "@mui/material/Typography";
+import { PageEnter } from "../ui";
+import { useAuth } from "../context/AuthContext";
+import DashAssistant from "../components/dashboard/DashAssistant";
+
+export default function Odin() {
+  const { hasPermission } = useAuth();
+
+  if (!hasPermission("ai.use")) {
+    return (
+      <PageEnter>
+        <Typography variant="body1" color="text.secondary">
+          Nemáte oprávnění pro AI asistenta.
+        </Typography>
+      </PageEnter>
+    );
+  }
+
+  return (
+    <PageEnter>
+      <Box sx={{ maxWidth: 920, mx: "auto" }}>
+        <DashAssistant />
+      </Box>
+    </PageEnter>
+  );
+}