fix(config): validate empty and overlapping port ranges

[FFFergie]

Summary

• Add config validation to reject empty monitoring.port_ranges.
• Add overlap detection for port ranges after sorting, with clear error messages.
• Add unit tests covering empty ranges, overlapping ranges (sorted/unsorted), and valid non-overlapping ranges.

Validation

• cargo test currently fails on upstream master due to pre-existing let-chain compatibility errors in unrelated files (src/app.rs, src/process/ports.rs, src/integrations/brew.rs, src/integrations/docker.rs).
• The new config tests are included in this PR but full test execution is blocked by that existing baseline issue.

Closes #33