fix: keep demo console table scrollable

[codeforester]

Summary

• keep the service panel horizontally scrollable at all widths below the 860px table minimum
• remove the intermediate-width clipping caused by overflow: hidden
• add a regression test for the responsive CSS contract

Validation

• mise run validate (isolated caches)
• mise exec -- bats tests/demo_console_test.bats
• git diff --check

Fixes #257