Remove divider line between left and right panels

[tarcode2004]

Description

The vertical divider line between the left navigation panel and the right aside panel doesn't look great visually. Remove it for a cleaner appearance.

Context

The Mantine AppShell aside component currently renders with withBorder. This border should be removed or replaced with a subtler visual separation (e.g., background color difference or shadow).

Acceptance Criteria

• The hard divider line between the left and right panels is removed
• The layout still feels visually organized without the divider
• No regression in the panel layout or spacing