Profile compare / diff

[nitin-1926]

Summary

Side-by-side compare of two profiles: skills, MCP servers, plugins, permissions, settings. Answers "why does profile A behave differently from B?".

We already compute the cascade + merged settings (CascadeService, DetailsService) — this diffs two of them.

Acceptance criteria

• Pick two profiles → columnar diff.
• Highlight added / removed / changed per asset category.
• Show provenance (host / global / profile) for each side.
• Empty/unknown profile handled gracefully.