[Application] default result overlay to peak density

[SilverSupplier]

Summary

• Default the results screen map overlay to Peak Density (previously Occupancy) and relabel the option "Peak Density", so the first results view matches the documented behavior.
• The remaining overlay options (Occupancy, Pressure, Hotspots, Bottlenecks, Cross Flow, None) are unchanged.

Area

• Engine
• Domain
• Application

Architecture Check

• I kept the dependency direction application -> domain -> engine.
• I did not add Qt UI code to src/domain.
• I did not add domain or application dependencies to src/engine.
• I used src/ as the include root.

Verification

• cmake --preset windows-debug
• cmake --build --preset build-debug
• ctest --preset test-debug
• Not run (reason below)

Built the Qt app in release: cmake --preset windows-release + cmake --build --preset build-release --target safecrowd_app -> succeeds. The change is a default selection and a label only.

Risks / Follow-up

• UI default only; no change to overlay computation or logic.