fix(dashboard): remove conflicting navigation borders

[RadnoK]

Summary

• Remove the sidebar divider where it collided with the framed workspace.
• Keep a deliberate 20px gutter beside the workspace while the sidebar is expanded; preserve existing collapsed layout.
• Remove the rounded inset accent stripe from the active navigation item.
• Remove the decorative line at the top of overview statistic cards.

Verification

• Dashboard build passes.
• Dashboard end-to-end suite passes (34 passed, 6 skipped).