Improve AVL plot interactions

[ghorn]

Stack: 2 of 4 in the AVL visualization QoL stack. Depends on PR 1 conceptually; opened against master because GitHub cannot target fork-only stack branches as upstream PR bases.

Summary

• Improve the eigenmode plot with mode classification labels, equal-axis scale lock, pan/box/touch zoom, CSV export, damping guides, and unstable-mode divergence display.
• Improve Trefftz plot readability with better axis/legend layout and richer hover markers, including mobile touch hover persistence.
• Convert viewer/eigen control hover help to custom aria-label/data-tooltip tooltips with instant display instead of native title tooltips.

Screenshots

Checks

• node --check js/dist/app.js
• npx playwright test --workers=1 js/playwright/eigenmode_panel.spec.js js/playwright/eigenmode_invalid_mass_message.spec.js js/playwright/trefftz_axis_label_layout.spec.js js/playwright/trefftz_hover_cursor.spec.js

[kennyjensen]

Mouse scroll zooming on the Eigenmode plot is broken by this.