fix: Settings tabs titles not clearly visible when using blank nextcloud them...

[Roshan931]

Fixes #409

Root cause

Tab container border colors ignore theme contrast

Changes

• Removed the use-theme inline 'border-top-color' override from the settings and edit-credential tab containers and switched .tab_container borders to the contrast-safe Nextcloud CSS variables --color-border / --color-primary-element (mirrored in the built css/app.css and js/templates.js artifacts).