Summary

• Fix Firefox scrollbar styling by using valid scrollbar-color pairs.
• Reserve scrollbar gutter space for table containers so scrollbars remain visible/consistent in Firefox.
• Keep existing WebKit scrollbar styling unchanged.

Bounty

• Addresses Panel scrollbars do not appear on Firefox #8.
• I requested assignment in the issue before opening this PR.

Testing

• Not run locally: this environment has Node.js but no npm binary available to install/run the project build.
• Change is limited to CSS in src/components/layout/table/Table.svelte.

AI Assistance Disclosure

This PR was prepared with AI assistance and manually reviewed before submission.