Add desktop skills management

[pmbstyle]

Summary

• Add desktop IPC and renderer support for listing, installing, enabling, disabling, and deleting skills.
• Add a Skills tab to the desktop dashboard with install controls, detail view, source links, and clearer status labels.
• Clean up the dashboard title/menu treatment and align the skill install placeholder text across desktop and web UI.

Validation

• npm run build in desktop
• git diff --check