connections-sidebar: vertical scroll on overflow

[xal3xhx]

Summary

• Add `overflow-y: auto` + `min-height: 0` to `.connections-sidebar` so the nav list scrolls when more plugins are installed than fit in the panel height.

Test plan

• Install enough plugins that the sidebar nav overflows 365px.
• Open Connections panel, verify a vertical scrollbar appears on the sidebar only.
• Verify non-overflow case still renders unchanged.