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/AdminApp.tsx

@@ -12,6 +12,7 @@ import { LoadingState } from "./ui";
 import Login from "./pages/Login";
 import Dashboard from "./pages/Dashboard";
 
+const Odin = lazy(() => import("./pages/Odin"));
 const Users = lazy(() => import("./pages/Users"));
 const Attendance = lazy(() => import("./pages/Attendance"));
 const AttendanceHistory = lazy(() => import("./pages/AttendanceHistory"));
@@ -82,6 +83,7 @@ export default function AdminApp() {
                   )}
                   <Route element={<AppShell />}>
                     <Route index element={<Dashboard />} />
+                    <Route path="odin" element={<Odin />} />
                     <Route path="users" element={<Users />} />
                     <Route path="attendance" element={<Attendance />} />
                     <Route