Add asset pipeline and professionalize theme

Introduce a Node build system for minified CSS/JS with content hashes,
add PWA support, i18n helpers, accessibility improvements, and more
translatable strings. Move theme configuration from bookmarks to
localStorage, add Playwright tests, and set up CI workflows.

shaarli-pro/install.html

@@ -11,7 +11,7 @@
             <div class="col-md-6 col-md-offset-3">
                 <form method="post" action="{$base_path}/install" name="installform" id="installform">
                     <div class="card">
-                        <div class="card-header">Welcome to your Shaarli</div>
+                        <div class="card-header">{'Welcome to your Shaarli'|t}</div>
                         <div class="card-body">
                             <p class="text-muted" style="margin-bottom: 1.5rem;">{'It looks like it\'s the first time you run Shaarli. Please configure it.'|t}</p>
                             <div class="form-group">