style: apply moonlit fog gradient background to body

Replace solid gray background with sophisticated gradient:
- Top-right radial gradient (white glow) at 70% 30%
- Bottom-left radial gradient (light steel blue) at 30% 70%
- Diagonal linear gradient from dark slate (#2c3e50) to light slate blue (#5f7995)
- Blend modes: soft-light, screen, normal for layered effect
- Filter: brightness(1.05) contrast(1.05) for enhanced visibility
- Background-attachment: fixed for parallax effect

Creates elegant moonlit atmosphere with subtle lighting effects.

static/css/main.css

@@ -22,7 +22,19 @@
 
 body {
     font-family: 'Quicksand', 'Source Sans Pro', -apple-system, system-ui, sans-serif;
-    background-color: var(--bg-gray);
+    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-attachment: fixed;
+    filter: brightness(1.05) contrast(1.05);
     color: rgb(41, 43, 44);
     line-height: 1.5;
     font-size: 16px;