Add conditional histogram UI view

[krystophny]

Fixes #22.

Summary

• replace the old hardcoded conditional histogram prototype with reusable Plotly helpers
• add a conditional histograms graph type to the main Dash UI
• reuse existing filter-table ranges to overlay filtered marginals against all samples
• make profit.ui import lazy so non-Dash helper modules do not require Dash at import time
• document the new UI graph type

Verification

• python -m pytest tests/unit_tests/ui/test_condhist.py -q
  • result: 4 passed in 0.44s
• python - <<'PY' ... import profit.ui as ui ... PY
  • result: True
• python -m compileall -q profit/ui/app.py profit/ui/condhist.py profit/ui/__init__.py tests/unit_tests/ui/test_condhist.py
  • result: passed with no output

Note: this container does not currently have the declared dash runtime package installed, so the focused tests cover the pure histogram logic and lazy package import without launching the Dash server.