Stabilize substitutions row alignment and dynamic sizing on class switch

[Copilot]

In 4.1, substitution rows started rendering with inconsistent indentation (notably first rows) and the substitutions container could keep stale sizing after switching classes. This change normalizes row layout constraints and class-label visibility logic so row geometry remains stable across data and class-context changes.

• Row layout normalization

  • Restore a fixed leading width for the period column to prevent horizontal drift between rows.
  • Use explicit leading alignment on the row container to avoid implicit centering effects.

• Width and sizing stability

  • Make each substitution row claim full available width (maxWidth: .infinity) so list cell sizing is recomputed consistently.
  • Keep horizontal distribution stable even when the class badge is conditionally hidden.

• Class-context correctness

  • Compare against primaryClass (active class) instead of klasses.first when deciding whether to render the trailing class badge.

HStack(alignment: .center) {
    Text("\(substitution.period).")
        .font(.title2)
        .fontWeight(.bold)
        .frame(width: 40, alignment: .leading)

    VStack(alignment: .leading, spacing: 2) { ... }

    Spacer(minLength: 0)
    if substitution.klassName != session.user?.primaryClass {
        Text(substitution.klassName).font(.callout)
    }
}
.frame(maxWidth: .infinity, alignment: .leading)