feat(openclaw): add default model button and fix sessionsApi export

- Add "Enable as Default" button to OpenClaw provider cards
- Place enable button before add/remove button for better UX
- Disable remove button when provider is set as default
- Add sessionsApi re-export from api barrel file to fix white screen
- Add i18n keys for default model feature (zh/en/ja)

src/App.tsx

@@ -198,6 +198,7 @@ function App() {
     switchProvider,
     deleteProvider,
     saveUsageScript,
+    setAsDefaultModel,
   } = useProviderActions(activeApp);
 
   const disableOmoMutation = useDisableCurrentOmo();
@@ -670,6 +671,9 @@ function App() {
                         activeApp === "claude" ? handleOpenTerminal : undefined
                       }
                       onCreate={() => setIsAddOpen(true)}
+                      onSetAsDefault={
+                        activeApp === "openclaw" ? setAsDefaultModel : undefined
+                      }
                     />
                   </motion.div>
                 </AnimatePresence>