feat: add Scanner background component

[clanzhang]

Description

Ports the <Scanner /> background component from react-bits to Vue 3.

Closes #178

What it does

A retro CRT scanner background built with OGL WebGL2 shaders:

• Vertical / horizontal / diagonal scan line sweep
• 3-color gradient with chromatic aberration
• Mouse-following glow interaction
• CRT scanlines, film grain, and vignette effects

Changes

File	Description
src/content/Backgrounds/Scanner/Scanner.vue	Core component (Vue 3 port, Composition API + <script setup>)
src/demo/Backgrounds/ScannerDemo.vue	Demo page with interactive controls (color pickers, sliders, direction select, mouse toggle)
src/constants/code/Backgrounds/scannerCode.ts	Usage code snippet
src/constants/Components.ts	Register scanner demo route
src/constants/Information.ts	Add Scanner metadata
src/constants/Categories.ts	Add to NEW array and Backgrounds sidebar

Dependencies

No new dependencies — ogl is already in package.json.

Checklist

• Component follows existing OGL background pattern (e.g. Aurora.vue)
• Demo page with color pickers, sliders, and prop table
• No new npm dependencies required
• Cleanup on unmount (cancelAnimationFrame, disconnect observers, lose WebGL context)
• Visibility-aware (pauses when tab hidden or component scrolled out of viewport)
• Build passes (npm run build)

[clanzhang]

@Utkarsh-Singhal-26