docs(branding): update logo, adopt NixOS 26.05 across docs and refresh screenshots

docs/development/api-reference.md

@@ -23,7 +23,7 @@ Returns high-level appliance status, health metrics, and active generation.
     "zfsPoolsCount": 1,
     "activeSharesCount": 3,
     "runningAppsCount": 2,
-    "osVersion": "NaxOS 24.11 (NixOS Vicuna)",
+    "osVersion": "NaxOS 26.05 (NixOS)",
     "nixosGeneration": 42
   }
 }