Pedagogy: add Hinton diagram + step slider to Gram-Schmidt notebook

Apply the repository's standard matrix-method visualization (Hinton from `60_linear_algebra_2/matshow.py`, paired with an ipywidgets step slider) to `40_linear_algebra_1/37_Gram_Schmidt.ipynb`.

## Scope

- Hinton diagram of the working `Q` matrix (orthonormal columns built one at a time) at each step.
- Optionally also Hinton the residual `aₖ − Σ projₑⱼ aₖ` to show the projection-removal visually.
- Slider over column index `k = 1…n`.

## Why

Gram-Schmidt is the algorithmic core of QR factorization. The teaching point is *orthogonalization* — each new column has its projections onto prior columns subtracted away. Hinton's size-encoding makes that subtraction-to-orthogonality visible per step in a way `matshow`/heatmap cannot. The orthonormal `Q` columns also display as same-magnitude blocks, which is satisfying confirmation of the result.

## Note

No standalone QR-iteration / QR-factorization notebook exists yet; Gram-Schmidt is the substrate. If a QR-iteration notebook is later added, it should follow the same Hinton + slider pattern (triangularization of `Aₖ` is the visual point there).