Fix blink root cause: eliminate position:fixed GPU compositing layers

Chrome promotes ALL position:fixed elements to GPU compositing layers for scroll
performance, regardless of whether they have animations. The body::before scanline
overlay (position:fixed, z-index:9999, full-viewport) and body::after watermark
(position:fixed) were both on GPU layers. Every CPU repaint from any hover state
change required a compositor re-blend pass → one-frame blink at compositor sync.

Fixes:
- Move scanlines from body::before (position:fixed) into body { background-image }
  — same visual, no separate element, no GPU layer promotion
- Set body::before { display:none } and body::after { display:none } in both
  dashboard.css and base.css
- Remove animation:matrix-rain from .stat-card:hover::before — background-position
  animation is not GPU-composited, caused CPU repaints every frame while hovered
  plus GPU texture uploads when animation started/stopped on cursor enter/exit
- Scope a { transition: all } → transition: color in base.css

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

assets/css/dashboard.css

@@ -96,28 +96,29 @@ body {
     margin: 0;
     padding: var(--spacing-md);
     background-color: var(--bg-primary);
-    color: var(--text-primary);
-    position: relative;
-}
-
-/* CRT Scanline Effect - Subtle retro terminal look */
-body::before {
-    content: '';
-    position: fixed;
-    top: 0;
-    left: 0;
-    width: 100%;
-    height: 100%;
-    background: repeating-linear-gradient(
+    /* Scanlines baked into body background — avoids position:fixed GPU layer */
+    background-image: repeating-linear-gradient(
         0deg,
-        rgba(0, 0, 0, 0.15),
+        rgba(0, 0, 0, 0.15) 0px,
         rgba(0, 0, 0, 0.15) 1px,
         transparent 1px,
         transparent 2px
     );
-    pointer-events: none;
-    z-index: var(--z-overlay);
-    animation: none;
+    color: var(--text-primary);
+    position: relative;
+}
+
+/* body::before was the CRT scanline overlay (position:fixed, z-index:9999).
+   Chrome promotes all position:fixed elements to GPU compositing layers, so
+   every hover-triggered CPU repaint caused a compositor re-blend blink.
+   Scanlines moved to body background-image above. */
+body::before {
+    display: none;
+}
+
+/* Suppress body::after binary text watermark (also position:fixed → GPU layer) */
+body::after {
+    display: none;
 }
 
 /* Screen Flicker Effect */
@@ -293,7 +294,6 @@ a:not(.btn):hover::after {
 
 .stat-card:hover::before {
     opacity: 1;
-    animation: matrix-rain 2s linear infinite;
 }
 
 @keyframes matrix-rain {