Fix bounce-rate chart percentage scaling

[benvinegar]

Summary

• use a conventional fixed 0–100% domain for the bounce-rate chart axis
• ensure a 46% bounce rate occupies 46% of the independent percentage axis
• add focused regression coverage for the bounce-rate domain
• leave tooltip content and ordering unchanged

Before and after

These Chromium captures use the actual TimeSeriesChart, Recharts, and repository CSS. The capture waited for chart animation to complete and verified stable, complete SVG paths before taking each screenshot.

For the 46% point:

• Before, 0–120% domain: occupied 38.33% of plot height
• After, 0–100% domain: occupies exactly 46% of plot height

The 100% point now reaches the top of the percentage plot, as expected. Full methodology, individual captures, geometry, checksums, and test output.

Validation

• focused TimeSeriesChart regression test: 1 passed
• server suite: 30 files, 175 tests passed
• build: passed
• typecheck: passed after generating build output
• lint: passed with existing warnings only
• git diff --check: passed
• independent code and evidence review: passed with no blockers

Fixes #128

This PR description was generated by Pi using OpenAI GPT-5.6 Sol

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.90%. Comparing base (fe226f6) to head (e8b1ab1).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #266   +/-   ##
=======================================
  Coverage   83.90%   83.90%           
=======================================
  Files          55       55           
  Lines        1882     1882           
  Branches      518      518           
=======================================
  Hits         1579     1579           
  Misses        267      267           
  Partials       36       36           

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