fix(css): give the install command its own top margin

[Sunrisepeak]

Summary

.cmd set only margin-bottom: 6px. Its top spacing came from .iface (margin: 16px 0 4px), the wrapper used on indexes whose plugin declares an interface line.

An index that declares none renders the install command directly after .lede (margin: 6px 0 0). With neither element contributing a margin between them, the description and the command touch:

A small, session-aware terminal UI for DeepSeek Harness
xlings install dsh:dsh-tui

That is a reader's report from dsh-index, whose packages are config layers rather than things you import, so no interface line is ever declared there.

Change

.cmd { margin: 14px 0 6px; }   /* was: margin-bottom: 6px */
.cmd + .cmd { margin-top: 6px; }
.iface .cmd { margin-top: 0; }

The command owns its top margin instead of borrowing an ancestor's.

Test plan

• python3 -m pytest tests/ -q → 45 passed (CSS-only change; no test asserts on spacing).
• Rebuilt dsh-index (71 packages) against this branch and loaded a package page: the lede and the install command are now separated, and pages that do declare an interface line are unchanged because .iface .cmd keeps its margin-top: 0.
• Consecutive commands (interface + alternates) stay at the original 6px via .cmd + .cmd.