Add dashboard skills page

[pmbstyle]

Summary

• Add a Skills page to the dashboard workspace navigation.
• Wire the page to the dashboard skills API for listing, installing, enabling, disabling, and deleting skills.
• Show skill names, descriptions, readiness, source/runtime details, requirements, and install feedback.

Validation

• npm run build
• npm run test
• git diff --check
• Browser QA against the served dashboard with empty and populated skill inventories, including install and disable flows.