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;

frontend/packages/dashboard/src/home.js

@@ -5,6 +5,9 @@
 //   2. fill the swarm/hive identity line when configured;
 //   3. render the shared server-warnings banner (top of every page).
 // No SSE — a portal doesn't need live updates.
+//
+// It also drives the decorative matrix-rain backdrop (#matrix-rain) —
+// see startMatrixRain() at the foot.
 
 import { renderServerWarnings } from './common.js';
 
@@ -48,3 +51,70 @@ async function init() {
 }
 
 init();
+
+// ── matrix-rain backdrop ─────────────────────────────────────────────
+// Classic falling-glyph "digital rain" on the #matrix-rain canvas, purely
+// decorative. Colours are resolved from the live stylix palette (--green
+// glyphs, --bg fade-trail) so a theme swap re-colours it; the CSS already
+// dims it + parks it behind the content. Stepped via setInterval (the
+// steppy cadence is the look and costs little CPU); paused while the tab is
+// hidden, and skipped entirely under prefers-reduced-motion.
+function startMatrixRain() {
+  const canvas = $('matrix-rain');
+  if (!canvas || !canvas.getContext) return;
+  if (window.matchMedia && window.matchMedia('(prefers-reduced-motion: reduce)').matches) return;
+  const ctx = canvas.getContext('2d');
+
+  // getComputedStyle on a custom property returns its *declared* value
+  // (e.g. "var(--base0B)") unresolved, so resolve to a concrete rgb() by
+  // probing a real `color` computation instead.
+  function resolveColor(varName, fallback) {
+    const probe = document.createElement('span');
+    probe.style.color = `var(${varName})`;
+    probe.style.display = 'none';
+    document.body.appendChild(probe);
+    const c = getComputedStyle(probe).color;
+    probe.remove();
+    return /^rgb/.test(c) ? c : fallback;
+  }
+  const glyphColor = resolveColor('--green', 'rgb(166, 227, 161)');
+  const fadeColor = resolveColor('--bg', 'rgb(30, 30, 46)')
+    .replace(/^rgb\(/, 'rgba(').replace(/\)$/, ', 0.09)');
+
+  const GLYPHS = 'アイウエオカキクケコサシスセソタチツテト0123456789:=*+-<>¦';
+  const CELL = 16; // glyph cell size (px)
+  let cols = 0;
+  let drops = [];
+
+  function resize() {
+    canvas.width = canvas.offsetWidth;
+    canvas.height = canvas.offsetHeight;
+    cols = Math.max(1, Math.ceil(canvas.width / CELL));
+    drops = Array.from({ length: cols }, () => Math.floor(Math.random() * -40));
+    ctx.font = `${CELL}px monospace`; // canvas reset on resize clears this
+  }
+
+  function tick() {
+    // Translucent bg wash leaves a fading trail behind each glyph.
+    ctx.fillStyle = fadeColor;
+    ctx.fillRect(0, 0, canvas.width, canvas.height);
+    ctx.fillStyle = glyphColor;
+    for (let i = 0; i < cols; i++) {
+      const ch = GLYPHS[(Math.random() * GLYPHS.length) | 0];
+      ctx.fillText(ch, i * CELL, drops[i] * CELL);
+      if (drops[i] * CELL > canvas.height && Math.random() > 0.975) drops[i] = 0;
+      drops[i]++;
+    }
+  }
+
+  let timer = null;
+  const play = () => { if (!timer) timer = setInterval(tick, 55); };
+  const pause = () => { if (timer) { clearInterval(timer); timer = null; } };
+
+  resize();
+  window.addEventListener('resize', resize);
+  document.addEventListener('visibilitychange', () => (document.hidden ? pause() : play()));
+  play();
+}
+
+startMatrixRain();

frontend/packages/dashboard/src/index.html

@@ -12,6 +12,13 @@
 </head>
 <body class="home-shell">
 
+  <!-- Decorative matrix-rain backdrop: a dimmed full-viewport canvas of
+       falling glyphs behind everything (home.js drives it). aria-hidden +
+       pointer-events:none so it's purely cosmetic; glyph/bg colours are
+       read from the stylix palette at runtime so a theme swap re-colours
+       it, and it's disabled under prefers-reduced-motion. -->
+  <canvas id="matrix-rain" aria-hidden="true"></canvas>
+
   <!-- H0M3: the menu hub. A plain grid of links to every top-level
        surface. This is the page served at `/` — the landing page — with
        the dashboard relocated to /dashboard.html. No tabbar / SSE —