Add shift-letter animations and a toggleable help overlay

[curtisahuang]

This PR introduces a new shift-key animation variant for letter-triggered effects and adds a glassy, toggleable help panel that explains how to interact with the page.

What changed:

• Animations

  • newEffect(letter, intensity, variant) now accepts a variant. When shift is held, letters spawn with a different animation variant (shift). Implemented by mapping variant to a different type selection.
  • The animation type for the shift variant is computed as (idx + 4) % 9 to differentiate from the normal variant.
  • Letter spawning now respects repeat and shiftKey for intensity and variant.

• Help overlay

  • Added a help container (#help) that describes how to play: move the mouse, press letters, use numbers to change background, and press ` to toggle help.
  • Help defaults to visible on load (helpVisible = true) and can be toggled with the backtick (`) key.
  • Implemented in code with applyHelpVisibility() and toggleHelp() to show/hide the help panel.
  • Updated HTML to include the help panel and CSS for a glassy, accessible panel that overlays the page.
  • Backtick key now toggles the help panel; 0–9 change the background; letters spawn effects with optional shift-variant animation.

• UI/CSS

  • Added styles for the glass-like help panel, including a subtle blur, translucent background, and smooth show/hide transitions.
  • Minor font/style tweak to improve canvas UI readability when the help panel is visible.

How this solves the task:

• Users can now trigger a distinct animation by holding Shift while pressing letters, giving a visual variety to the spawned effects.
• A built-in, ready-to-use help panel guides new users on how to interact with the page, defaulting to visible, and can be hidden with the ` key as requested.

https://cosine.wtf/cosine-stg/cat-coded-page/task/e0nj711tv7bz
https://cosine.wtf/gh/CosineAI/cat-coded-page/pull/2
Author: Curtis Huang