feat: persist active settings tab in URL hash (v55)

URL gains &stab=<panel> when settings are open (e.g. #...&settings=1&stab=accounts).
Tab switches update the URL; reload reopens settings on the same tab.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/js/version.js

@@ -1,2 +1,2 @@
 // Increment APP_VERSION with every code change
-export const APP_VERSION = 'v54';
+export const APP_VERSION = 'v55';