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/Dashboard.tsx

@@ -22,7 +22,6 @@ import DashSessions from "../components/dashboard/DashSessions";
 import DashTodayPlan, {
   type TodayPlan,
 } from "../components/dashboard/DashTodayPlan";
-import DashAssistant from "../components/dashboard/DashAssistant";
 
 const API_BASE = "/api/admin";
 
@@ -237,9 +236,6 @@ export default function Dashboard() {
         </Typography>
       </Box>
 
-      {/* AI Assistant widget */}
-      {hasPermission("ai.use") && <DashAssistant />}
-
       {/* 2FA Required Banner */}
       {user?.require2FA && !user?.totpEnabled && (
         <Card