feat: server admin password reset via tenant portal

- POST /api/tenant/server/admin-password — resets server's built-in
  admin password via M2M API call to the tenant's server
- Settings page: "Server Admin Password" card
- ServerApiClient.resetServerAdminPassword() calls server's password
  reset endpoint with M2M token

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/api/tenant-hooks.ts

@@ -94,6 +94,12 @@ export function useRemoveTeamMember() {
   });
 }
 
+export function useResetServerAdminPassword() {
+  return useMutation<void, Error, string>({
+    mutationFn: (password) => api.post('/tenant/server/admin-password', { password }),
+  });
+}
+
 export function useChangeOwnPassword() {
   return useMutation<void, Error, string>({
     mutationFn: (password) => api.post('/tenant/password', { password }),