dashboard(home): matrix-rain backdrop on the H0M3 page

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.

frontend/packages/dashboard/src/home.css

@@ -13,6 +13,21 @@ body.home-shell {
   min-height: 100vh;
 }
 
+/* Matrix-rain backdrop (driven by home.js). Fixed, full-viewport, dimmed,
+   and behind every other element (z-index below the auto-stacked content).
+   Decorative only — pointer-events off so it never intercepts clicks. The
+   low opacity keeps the menu tiles legible over it; glyph + fade colours
+   are themed from the stylix palette in JS. */
+#matrix-rain {
+  position: fixed;
+  inset: 0;
+  width: 100%;
+  height: 100%;
+  z-index: -1;
+  opacity: 0.16;
+  pointer-events: none;
+}
+
 .home-header {
   text-align: center;
   padding: 1.5em 0 0.5em;