refactor: modularize into ES6 modules

Split monolithic index.html into:
- js/constants.js - gate config, colors, dimensions
- js/state.js     - shared application state
- js/gates.js     - evaluation logic, port geometry
- js/renderer.js  - canvas drawing
- js/waveform.js  - GTKWave-style signal viewer
- js/simulation.js - clock tick engine
- js/events.js    - mouse, keyboard, UI handlers
- js/app.js       - entry point
- css/style.css   - all styles

Dockerfile

@@ -1,3 +1,5 @@
 FROM nginx:alpine
-COPY index.html /usr/share/nginx/html/index.html
+COPY index.html /usr/share/nginx/html/
+COPY css/ /usr/share/nginx/html/css/
+COPY js/ /usr/share/nginx/html/js/
 EXPOSE 80