refactor(config): Update GPTEL model and backend configuration

Update the default model to 'gemma-4-26B' and adjust the list of
available models for the OpenAI backend configuration.

configs/init-ai.el

@@ -42,14 +42,16 @@
                                   "qwen3.5:4b"
                                   "su_robin/gemma-4-E4B-it-Q4_K_M:latest")))
 
-  (setq gptel-model 'gemma-4-e2b
+  (setq gptel-model 'gemma-4-26B
         gptel-backend
-        (gptel-make-openai "llama-cpp"
+        (gptel-make-openai "llama.cpp"
           :stream t
           :protocol "http"
           :host "127.0.0.1:1945"
-          :models '("gemma-4-e4b"
-                    "gemma-4-e2b")))
+          :models '("gemma-4-E2B"
+                    "gemma-4-E4B"
+                    "gemma-4-26B"
+                    "Qwen3.5-0.8B")))
 
   (setq gptel-default-mode 'org-mode
         gptel-prompt-prefix-alist