feat(ui): add ARIA attributes for accessibility improvements

Add aria-hidden="true" to decorative Font Awesome icons across templates,
aria-label to icon-only buttons, aria-live to dynamic content regions,
aria-labelledby to modals, aria-expanded to toggle buttons, scope="col"
to table headers, and aria-label to tables. Also improve alt text on
500.html error image and add aria-label to password toggle buttons.

Templates updated: settings.html, credentials.html, status_dashboard.html,
file_view.html, file_detail.html, index.html, queue_dashboard.html,
audit_log.html, 500.html

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

frontend/templates/500.html

@@ -7,7 +7,7 @@
   <div class="mx-auto max-w-3xl py-12">
     <img
       src="/static/explosion.jpg"
-      alt="Server Explosion"
+      alt="Illustration of a server error"
       class="mx-auto rounded-xl shadow-xl h-[300px] w-full object-cover"
     />
   </div>