feat(health): config health dashboard

[SurviveM]

Requirement

Auto-detect on startup: API key validity, provider reachability, model availability, session file integrity. Dashboard shows green/yellow/red status with actionable fix suggestions.

Verification

1. Intentionally set wrong API key in config -> refresh dashboard -> see red warning + specific fix suggestion
2. Fix the key -> refresh -> status turns green
3. Delete a session file manually -> dashboard detects integrity issue -> offers restore from trash