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/components/dashboard/DashAssistant.tsx

@@ -361,7 +361,7 @@ export default function DashAssistant() {
             AI
           </Box>
           <Typography variant="subtitle1" sx={{ fontWeight: 600 }}>
-            Asistent
+            Odin
           </Typography>
         </Box>
         <Box sx={{ display: "flex", alignItems: "center", gap: 1.5 }}>
@@ -393,8 +393,8 @@ export default function DashAssistant() {
           display: "flex",
           flexDirection: "column",
           gap: 1,
-          minHeight: 120,
-          maxHeight: 360,
+          minHeight: 200,
+          maxHeight: 480,
           overflowY: "auto",
           mb: 1.5,
           p: 1,