Feature/dimentionalyreduction exercise for students to complete matrix mystics

[VigneshRapolu]

Summary

Adds an interactive dimensionality-reduction (PCA) exercise to the student
dashboard, as a new activity for the Matrix Mystics Module 6 topics — which
currently have no worked activity attached to them.

What this adds

• New PCAInteractive component in client/src/main.jsx.
• New "PCA Exercise" tab on the student dashboard, alongside the existing
  SP Bank, Polls, and Leaderboard tabs.
• A small editable table (student -> two attendance-style values). Editing
  any value live-updates:
  • the scatter plot of all points
  • the mean and covariance
  • the best-fit direction (PC1)
  • each point's projection onto that direction
  • the "variance explained" percentage

Scope

• Client-side only. No backend routes, no database queries, no new
  dependencies.
• Starting values are illustrative placeholders, not real student data —
  fully editable and self-contained, so it's reproducible without any DB
  connection.
• Purely additive: no changes to scoring, SP calculation, or any other
  existing feature/route.

Out of scope (left for a future PR)

• Wiring this up to real Spurti attendance/poll data. That needs a
  separate decision on which fields to expose and how, so it's left out
  here to keep this PR small and self-contained.

Testing

Verified locally: editing table values updates the plot, mean, covariance,
PC1 direction, and variance-explained % correctly with no console errors.