Remove react-plotly.js once all migratable charts are done

Summary

Once all feasible chart components are migrated away from Plotly (see #609, #663), remove react-plotly.js and plotly.js from the dependency tree to reduce bundle size significantly.

Prerequisites

• #663 merged (16 charts migrated to Recharts)
• Remaining Plotly-only charts either migrated to alternatives or confirmed as permanently needing Plotly
• Blog/notebook Plotly JSON rendering evaluated for alternatives

Current Plotly footprint

plotly.js adds ~3MB to the bundle (minified). Even with code splitting, it's a significant chunk. If we can eliminate all Plotly usage, the savings are substantial.

What needs to happen

1. Migrate or confirm each remaining Plotly chart (tracked separately)
2. If blog/notebook Plotly JSON must stay, consider lazy-loading Plotly only for those routes
3. Remove react-plotly.js and plotly.js from package.json
4. Remove Plotly mock from test setup (vitest.setup.mjs)
5. Update .claude/skills/chart-standards.md to remove Plotly references

Context

Related: #609, #663

Start by checking prerequisites #663 and #609, then audit the remaining Plotly-only charts and the blog/notebook Plotly JSON rendering. Review package.json, vitest.setup.mjs, and .claude/skills/chart-standards.md. Done means remaining charts are migrated or confirmed, Plotly dependencies and mocks are removed, and Plotly references in the chart standards are updated.