refactor: Reorganize config with use-package and optimize startup hooks

- Split monolithic init-package.el into focused modules
- Convert eager `:init` blocks to lazy `:hook`
- Consolidate UI settings and defer theme loading.
- Remove duplicated config between GUI and terminal paths.

lisp/init-gptel.el

@@ -111,9 +111,8 @@ from the last 1500 characters of the buffer."
     (define-key gptel-mode-map (kbd "C-c C-s") #'my-gptel-save-session)))
 
 (use-package gptel-agent
-  :init
-  (with-eval-after-load 'gptel
-    (require 'gptel-agent))
+  :after gptel
+  :demand t
   :config
   (add-to-list 'gptel-agent-dirs "~/.emacs.d/lisp/" t)
   (gptel-agent-update))