feat(ai): Add Gemini backend with Ollama fallback

Prioritize Gemini backend if GEMINI_KEY enviroment variable is present,
otherwise fallback to the local Ollama instance.

configs/init-ai.el

@@ -12,17 +12,22 @@
 ;; Cliente LLM (ollama, chatgpt, gemini, etc.)
 (use-package gptel
   :config
-  ;; (setq gptel-model 'gemma3:4b
-  ;;       gptel-backend (gptel-make-ollama "Ollama"
-  ;;                       :host "localhost:11434"
-  ;;                       :stream t
-  ;;                       :models '("mistral:latest"
-  ;;                                 "deepseek-r1:1.5b"
-  ;;                                 "deepcoder"
-  ;;                                 "dolphin-llama3:latest"
-  ;;                                 "gemma3:4b"
-  ;;                                 "llava:latest"))
-        gptel-default-mode 'org-mode
+  (if (getenv "GEMINI_KEY")
+      (setq gptel-model 'gemini-2.5-flash
+            gptel-backend (gptel-make-gemini "Gemini"
+                            :key (getenv "GEMINI_KEY")
+                            :stream t))
+    (setq gptel-model 'gemma3:4b
+          gptel-backend (gptel-make-ollama "Ollama"
+                          :host "localhost:11434"
+                          :stream t
+                          :models '("mistral:latest"
+                                    "deepseek-r1:1.5b"
+                                    "deepcoder"
+                                    "dolphin-llama3:latest"
+                                    "gemma3:4b"
+                                    "llava:latest"))))
+  (setq gptel-default-mode 'org-mode
         gptel-prompt-prefix-alist
         '((markdown-mode . "# User\n\n")
           (org-mode . "* User\n\n")