feat(skills): add marketplace tabs and install flow

[golemcore1]

Summary

• added backend skill marketplace support with configurable GitHub source, catalog endpoint, and install endpoint
• added dashboard marketplace UI under the existing Skills menu with Installed/Marketplace tabs and URL query sync (/skills?tab=marketplace)
• extracted local skills editor/list into LocalSkillsPanel and added marketplace cards/install flow with query invalidation
• fixed mobile topbar hamburger icon visibility and alignment
• added controller and service tests for marketplace catalog and install scenarios

Notes

• dashboard npm run lint passes
• backend tests were not runnable in this environment because private dependencies are unavailable: me.golemcore.plugins:golemcore-plugin-extension-api:1.0.0, me.golemcore.plugins:golemcore-plugin-runtime-api:1.0.0

[sonarqubecloud]

Quality Gate failed

Failed conditions
1 Security Hotspot
75.7% Coverage on New Code (required ≥ 80%)

See analysis details on SonarQube Cloud