fix(sidebar): use semantic border color token

[NBN7]

Why this change matters

This update ensures that the sidebar border uses the design system’s semantic color token instead of relying on implicit or default border colors.

Previously, the component only defined border placement (border-r, border-l) without explicitly setting the border color. This made the final appearance dependent on inherited styles or browser defaults, which can lead to inconsistencies across themes (especially when using CSS variables for theming).

By adding border-sidebar-border, we:

• Align with the design system: The sidebar now consistently uses the intended --sidebar-border token.
• Improve theme accuracy: Colors update correctly when switching or customizing themes (e.g., via CSS variables like --sidebar-border).
• Avoid implicit styling: Removes reliance on fallback or inherited border colors, making the component more predictable.
• Ensure visual consistency: Matches other components that already rely on semantic border tokens.

In short, this change makes the component fully theme-aware and consistent with the rest of the system.

[vercel]

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