fix(analysis): show only the density histogram during Peek Negative - #911

Summary

• Peek Negative shows the scan before any curve ran, but the Analysis chart kept
  showing the last real render's curve, output histogram and zone strip as if
  they described what's on screen — misleading, since none of that applies to
  an un-inverted, un-metered negative.
• The chart now hides the curve, output histogram, zone traces and clip marks
  while peeking, mirroring how Peek Flat Scan already hides the zone strip for
  the same reason. It's also a better look at the density histogram itself:
  with nothing else drawn behind it, the raw scan's own tonal spread is easier
  to read.
• The density histogram (what the scan itself contains, before the curve) is
  unaffected either way and stays visible throughout.

Test plan

• make all (lint + type check + full test suite)
• Added tests/test_negative_peek_analysis_chart.py and a step-wedge-hiding
  case in tests/test_step_wedge_widget.py
• Verified live in the app: loaded a frame, toggled Peek Negative, confirmed
  the curve/output histogram/zone strip/step wedge disappear and only the
  density histogram remains, then confirmed toggling back restores the
  normal Analysis chart

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