docs: align README and CONTRIBUTING with actual Dashboard, Deep Cleanup and test layout

[laurentiu021]

Summary

Documentation accuracy pass from the modernization audit. Brings user-facing docs in line with what the code actually does. docs: so no release.

Changes

• README — Dashboard: added the Real-time vitals (CPU/RAM/GPU at 300 ms while visible, polling auto-pauses) and Recent Activity features. Both exist in DashboardViewModel but were undocumented.
• README — Large files finder: demoted from a top-level ### section to a #### sub-item of Deep Cleanup, since it has no separate tab/ViewModel — it is driven by DeepCleanupViewModel via LargeFileScanner. The old heading implied a standalone tab.
• CONTRIBUTING — Running tests: added SysManager.IntegrationTests to the full-run commands and clarified that integration + UI tests run locally only (CI runs the unit tests).

Notes

• Verified the README "55 tabs / 30 implemented / 25 WIP" claim against the code (25 Wip* placeholder ViewModels) — accurate, left unchanged.
• The historical CHANGELOG "2281 tests" line (v1.7.20) was left intact — it is an immutable historical entry that was true at that release, not a current claim.

Verification

• Docs-only; internal links unaffected (the changed heading was not a link target).