Visualize waveform decomposition/synthesis.
Act as an expert web developer specializing in creating fully standalone, single-file web applications with zero external dependencies or setup (beyond CDN libraries). Your task is to create the complete HTML, CSS3 (inline `<style>`), and modern ES6 JavaScript (inline `<script>`) code for the following application, delivered as a single, self-contained HTML file: **App Title:** Fourier Series Explorer **Main Purpose:** Visualize waveform decomposition/synthesis. **Key Features to Implement:** * Select/draw waveform * Harmonic controls * Animation of components/sum * Frequency spectrum chart * Ability to handle user input via interactive controls (sliders, forms, buttons, drag-drop, etc., as appropriate).
Guide for creating a visually appealing web application with modern design principles.
