style: replace gradient with dark circuit board grid background

Replace moonlit fog gradient with clean tech-inspired grid pattern:
- Base color: #171717 (dark charcoal)
- Layered grid lines using linear gradients
- Large grid: 50px × 50px (#171717 lines)
- Fine grid: 10px × 10px (#262626 lines)
- Fixed attachment for consistent appearance while scrolling

Creates subtle circuit board aesthetic with dual-layer grid system.

static/css/main.css

@@ -22,19 +22,14 @@
 
 body {
     font-family: 'Quicksand', 'Source Sans Pro', -apple-system, system-ui, sans-serif;
-    background:
-        radial-gradient(circle at 70% 30%, rgba(255, 255, 255, 0.1) 0%, transparent 50%),
-        radial-gradient(circle at 30% 70%, rgba(176, 196, 222, 0.15) 0%, transparent 50%),
-        linear-gradient(135deg,
-            #2c3e50 0%,
-            #3a506b 25%,
-            #435e79 50%,
-            #516b87 75%,
-            #5f7995 100%
-        );
-    background-blend-mode: soft-light, screen, normal;
+    background-color: #171717;
+    background-image:
+        linear-gradient(90deg, #171717 1px, transparent 1px),
+        linear-gradient(180deg, #171717 1px, transparent 1px),
+        linear-gradient(90deg, #262626 1px, transparent 1px),
+        linear-gradient(180deg, #262626 1px, transparent 1px);
+    background-size: 50px 50px, 50px 50px, 10px 10px, 10px 10px;
     background-attachment: fixed;
-    filter: brightness(1.05) contrast(1.05);
     color: rgb(41, 43, 44);
     line-height: 1.5;
     font-size: 16px;