Modernize JavaScript to ES2015+ (const/let, arrows, templates) and refresh HTML/assets

[venku122]

Motivation

• Bring legacy code up to modern JavaScript style and safer globals to improve readability and ease future maintenance.
• Replace outdated patterns (lots of var, function expressions, string concatenation) with const/let, arrow functions and template literals.
• Tidy up WebGL/shader and utility helpers and fix minor HTML markup and font link issues.

Description

• Replaced many var usages with const/let and converted callbacks to arrow functions across js/* files to adopt ES2015+ idioms (js/main.js, js/keys.js, js/loader.js, js/audioHandler.js, js/emitter.js, js/rocket.js, js/setup.js, js/shaderUtilities.js, js/utilities.js).
• Modernized event handlers and logging (template strings), improved image loader counting with Object.keys(sources).length, and made small control-flow/loop variables block-scoped (let).
• Refactored the emitter/particle code to use clearer declarations and template strings for color composition, and updated audio handling to use const/let and arrow callbacks for XMLHttpRequest decode handling.
• Hardened global usage by assigning app to window.app where appropriate to make globals explicit, and updated a number of small API usages to be consistent with modern code style.
• Cleaned up WebGL shader utility file: switched kernels and buffers to const where appropriate, fixed canvas sizing and timing variable declarations, and converted callback countdown to an arrow function.
• Minor HTML fixes in rocketLander.html: corrected lang attribute, replaced obsolete Google Fonts URL with fonts.googleapis.com/css2, and fixed malformed <canvas id='gl'> tag.

Testing

• Launched a local static server with python -m http.server 8000 and exercised the UI by opening rocketLander.html via an automated Playwright script, which produced a screenshot at artifacts/rocket-lander.png; the page loaded and the screenshot run completed successfully.
• No unit tests were present or executed as part of this change set.

---

Codex Task