Add Fourier series explorer

[jbkunst]

Summary

• adds a new fourier-series Shinylive app based on app-template
• approximates quadratic, polynomial, periodic signal, and floor functions on [0, 10]
• uses a 2×2 Highcharter layout for approximation, leading components, spectrum, and reconstruction quality
• shows up to five leading components, with line width scaled by Fourier amplitude
• updates all charts with highchartProxy()
• keeps app.R at 267 lines

Notes

• no screenshot or build files are included; the existing gallery workflow handles the Shinylive export
• reconstruction quality is the percentage of target variation recovered by the cumulative Fourier approximation