dashboard(home): harden matrix-rain fade-trail colour parsing

Follow-up to the matrix-rain backdrop (merged): address argus's review nit
on the fadeColor construction. It string-rewrote `rgb(...)`→`rgba(...)`,
which silently fills the trail solid if getComputedStyle ever returns a
non-`rgb(r, g, b)` form. Extract r,g,b numerically and build the rgba
explicitly so the trail always fades regardless of the returned format
(same rgb fallback end-to-end). CSS/JS-only, home page.

frontend/packages/dashboard/src/home.js

@@ -78,8 +78,14 @@ function startMatrixRain() {
     return /^rgb/.test(c) ? c : fallback;
   }
   const glyphColor = resolveColor('--green', 'rgb(166, 227, 161)');
-  const fadeColor = resolveColor('--bg', 'rgb(30, 30, 46)')
-    .replace(/^rgb\(/, 'rgba(').replace(/\)$/, ', 0.09)');
+  // Fade-trail = the bg at low alpha. Pull r,g,b numerically rather than
+  // string-rewriting the rgb() — robust to whatever rgb()/rgba() spacing
+  // getComputedStyle hands back, so the trail always fades (never fills
+  // solid) even if the format shifts.
+  const bgRGB = resolveColor('--bg', 'rgb(30, 30, 46)').match(/(\d+)[^\d]+(\d+)[^\d]+(\d+)/);
+  const fadeColor = bgRGB
+    ? `rgba(${bgRGB[1]}, ${bgRGB[2]}, ${bgRGB[3]}, 0.09)`
+    : 'rgba(30, 30, 46, 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)