Adds the falling-glyph "digital rain" effect the operator asked for as a
dimmed background on the home hub.
- index.html: a decorative full-viewport <canvas id="matrix-rain">
(aria-hidden, pointer-events:none) as the first body child.
- home.css: fixes it behind the content (z-index:-1, inset:0) at low
opacity (0.16) so the menu tiles stay legible — the "dimmed bg".
- home.js: startMatrixRain() drives the classic stepped digital rain.
Glyph + fade-trail colours are RESOLVED FROM THE STYLIX PALETTE at
runtime (--green glyphs, --bg trail; resolved via a probe element
since getComputedStyle returns custom props unresolved), so a theme
swap re-colours it rather than hardcoding matrix-green. Stepped via a
55ms interval, paused while the tab is hidden, and skipped entirely
under prefers-reduced-motion.
CSS/JS only, home page only.
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