feat: replace Recharts dashboard chart with visx area chart

Integrated 21st.dev AreaChart component with animated crosshair,
spring-based tooltip, and date ticker. Uses brand orange for the
line/fill with dark-only CSS variables.

styles/globals.css

@@ -18,6 +18,18 @@
     --chart-grid: #262626;
     --chart-axis: #737373;
 
+    /* * visx area chart tokens (dark-only) */
+    --chart-background: #0a0a0a;
+    --chart-foreground: #404040;
+    --chart-foreground-muted: #a3a3a3;
+    --chart-line-primary: #FD5E0F;
+    --chart-line-secondary: #737373;
+    --chart-crosshair: #404040;
+    --chart-label: #a3a3a3;
+    --chart-marker-background: #262626;
+    --chart-marker-border: #404040;
+    --chart-marker-foreground: #fafafa;
+
     /* * shadcn-compatible semantic tokens (dark-only) */
     --background: 10 10 10;
     --foreground: 250 250 250;