feat(players): overlay skill history on ELO chart

[garrappachc]

Summary

• Adds a skill layer on top of the ELO history chart so admins can correlate ELO changes with manual skill adjustments
• Skill is rendered as a dashed white stepped line on a right-side Y-axis, aligned to existing ELO data points
• The overlay only appears when the selected class has admin-set skill history; charts for players without skill data are unchanged

How it works

For each ELO data point, buildSkillData finds the most recent skillHistory entry whose at date is ≤ the game date, producing a parallel array of number | null. This gives a step-function that stays flat between admin changes and jumps when skill is updated.

Test plan

• Open a player profile with skill history set — verify the dashed skill line appears on the ELO chart with a right-side axis
• Switch between class tabs — verify skill line updates correctly per class
• Switch between "Over time" / "By game #" modes — verify both axes render correctly
• Open a player profile with no skill history — verify chart looks identical to before (no second axis, no legend)

🤖 Generated with Claude Code

[github-actions]

Playwright test results

 199 passed

Details

 199 tests across 29 suites
 8 minutes, 33 seconds
 ef40cf4