perf(css): fix rendering slowdown on low-power hardware (Pi5)

- Remove backdrop-filter: blur(5px) from .card and .panel — on ARM/Linux
  Chromium this is software-rendered, causing severe CPU overhead at 42+
  instances. The opaque surface gradient makes blur imperceptible anyway.
- Remove inset vignette box-shadow from .panel added in 51c1014
- Rewrite panel-pulse keyframes to animate opacity only (was box-shadow,
  which triggers CPU repaint every frame; opacity is compositor-only)
- Gate body::before and .visuals-container::after scanline pseudo-elements
  under [data-animations="off"] — the toggle was blind to both
- Gate panel-indicator pulse under [data-animations="off"] for consistency

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

static/css/index.css

@@ -7345,6 +7345,11 @@ body::before {
     animation: none !important;
 }
 
+[data-animations="off"] body::before,
+[data-animations="off"] .visuals-container::after {
+    display: none;
+}
+
 /* ============================================
    VISUAL REFRESH OVERRIDES
    ============================================ */