feat: add in-browser histogram panel using jsroot (#840)

[rx18-eng]

Summary

Adds a histogram panel to the toolbar that shows invariant mass distributions live using jsroot. this is for the masterclass workflow (#840) - students tag particles, compute mass values, and the histogram fills up in real time so
they can actually see the physics (Z peak at 91 GeV etc).

uses the event bus (on/emit) i added to EventDisplay so the histogram subscribes to result-recorded events. config is experiment-agnostic ATLAS default included with reference lines for J/ψ, Z, and H.

Features Implemented:

• event bus on EventDisplay (on/emit + cleanup)
• HistogramConfig interface + ATLAS default config
• histogram panel components (CDK overlay pattern, same as collections-info)
• jsroot dark theme integration (gStyle + post-draw DOM fixes)
• reference hint lines aligned to jsroot pad margins
• reset + CSV export

Test Plan

• open /atlas, click histogram icon in toolbar
• run in console: window.EventDisplay.emit('result-recorded', {mass: 91.2})
• add more events, bars should cluster around Z peak
• test reset and export buttons

I have also uploaded a video of the same below with random 100 events:

Recording.2026-03-21.233005.mp4

closes #840

[rx18-eng]

hey @sponce @EdwardMoyse , this adds the histogram panel from #840. @cohm mentioned he wants to test this with students in Stockholm on Monday so it would be great if you could take a quick look before then. happy to make changes if anything needs fixing!