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/export.bookmarks.html

@@ -4,7 +4,7 @@
      It will be read and overwritten.
      Do Not Edit! -->{ignore}The RainTPL loop is formatted to avoid generating extra newlines{/ignore}
 <TITLE>{$pagetitle}</TITLE>
-<H1>Shaarli export of {$selection} bookmarks on {$date}</H1>
+<H1>{'Shaarli export of'|t} {$selection} {'bookmarks on'|t} {$date}</H1>
 <DL><p>{loop="links"}
 <DT><A HREF="{$value.url}" ADD_DATE="{$value.timestamp}" PRIVATE="{$value.private}" TAGS="{$value.taglist}">{$value.title}</A>{if="$value.description"}{$eol}<DD>{$value.description}{/if}{/loop}
 </DL><p>