refactor: improve width handling and introduce CSS variables (#678)

* refactor: update page width handling and CSS structure

- Changed the default page width from 1280px to 80rem in hugo.yaml.
- Replaced dynamic page width partials with a new CSS class 'hextra-max-page-width' across multiple layout files for consistency.
- Introduced a new head-config-css.html partial for managing CSS styles related to page width.
- Removed the outdated page-width utility partial to streamline the codebase.

* feat: introduce CSS variables for layout widths and update footer/navbar styles

- Added a new configs.css file to define CSS variables for page, navbar, and footer widths.
- Updated footer and navbar partials to utilize the new CSS classes for consistent width management.
- Refactored head-config-css.html to include the new navbar width variable.
- Enhanced the overall styling structure for better maintainability and responsiveness.

* Refactor: Rename configs.css to variables.css and update references

Remove head-config-css.html and update references to use variables.css instead of configs.css

* Update assets/css/variables.css

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update layouts/partials/head.html

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

layouts/partials/navbar.html

@@ -16,7 +16,7 @@
 <div class="nav-container hx:sticky hx:top-0 hx:z-20 hx:w-full hx:bg-transparent hx:print:hidden">
   <div class="nav-container-blur hx:pointer-events-none hx:absolute hx:z-[-1] hx:h-full hx:w-full hx:bg-white hx:dark:bg-dark hx:shadow-[0_2px_4px_rgba(0,0,0,.02),0_1px_0_rgba(0,0,0,.06)] hx:contrast-more:shadow-[0_0_0_1px_#000] hx:dark:shadow-[0_-1px_0_rgba(255,255,255,.1)_inset] hx:contrast-more:dark:shadow-[0_0_0_1px_#fff]"></div>
 
-  <nav class="hx:mx-auto hx:flex hx:items-center hx:justify-end hx:gap-2 hx:h-16 hx:px-6 {{ $navWidth }}">
+  <nav class="hextra-max-navbar-width hx:mx-auto hx:flex hx:items-center hx:justify-end hx:gap-2 hx:h-16 hx:px-6">
     <a class="hx:flex hx:items-center hx:hover:opacity-75 hx:ltr:mr-auto hx:rtl:ml-auto" href="{{ $logoLink }}">
       {{- if (.Site.Params.navbar.displayLogo | default true) }}
         <img class="hx:mr-2 hx:block hx:dark:hidden" src="{{ $logoPath | relURL }}" alt="{{ .Site.Title }}" height="{{ $logoHeight }}" width="{{ $logoWidth }}" />