Add particle background effect

- Pure CSS/JS particle animation behind all content
- Multiple layers with different speeds for depth effect
- Theme-aware: white particles on dark mode, purple on light mode
- Low opacity for subtle, non-distracting ambiance
- Uses box-shadow technique for performance

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/components/Layout.tsx

@@ -2,6 +2,7 @@ import { ReactNode, useState, useEffect, useRef } from 'react';
 import { Link, useNavigate } from 'react-router-dom';
 import { useAuth } from '../hooks/useAuth';
 import NotificationBell from './NotificationBell';
+import ParticleBackground from './ParticleBackground';
 
 interface LayoutProps {
   children: ReactNode;
@@ -293,6 +294,8 @@ export default function Layout({ children }: LayoutProps) {
         }
       `}</style>
 
+      <ParticleBackground />
+
       <nav className="navbar">
         <div className="navbar-content">
           <Link to="/" className="navbar-brand">

frontend/src/components/ParticleBackground.tsx

@@ -0,0 +1,108 @@
+import { useMemo } from 'react';
+
+// Generate random box-shadow particles
+function generateParticles(count: number, spacing: number, color: string): string {
+  const shadows: string[] = [];
+  for (let i = 0; i < count; i++) {
+    const x = Math.floor(Math.random() * spacing);
+    const y = Math.floor(Math.random() * spacing);
+    shadows.push(`${x}px ${y}px ${color}`);
+  }
+  return shadows.join(', ');
+}
+
+interface ParticleLayerProps {
+  count: number;
+  size: number;
+  duration: number;
+  color: string;
+  spacing: number;
+}
+
+function ParticleLayer({ count, size, duration, color, spacing }: ParticleLayerProps) {
+  const boxShadow = useMemo(() => generateParticles(count, spacing, color), [count, spacing, color]);
+  const boxShadowAfter = useMemo(() => generateParticles(Math.floor(count * 0.8), spacing, color), [count, spacing, color]);
+
+  return (
+    <div
+      className="particle-layer"
+      style={{
+        position: 'absolute',
+        top: 0,
+        left: 0,
+        width: `${size}px`,
+        height: `${size}px`,
+        background: 'transparent',
+        boxShadow,
+        borderRadius: '50%',
+        animation: `particleFloat ${duration}s linear infinite`,
+      }}
+    >
+      <div
+        style={{
+          position: 'absolute',
+          top: `${spacing}px`,
+          left: 0,
+          width: `${size}px`,
+          height: `${size}px`,
+          background: 'transparent',
+          boxShadow: boxShadowAfter,
+          borderRadius: '50%',
+        }}
+      />
+    </div>
+  );
+}
+
+export default function ParticleBackground() {
+  const spacing = 2000;
+
+  return (
+    <div className="particle-background">
+      <style>{`
+        .particle-background {
+          position: fixed;
+          top: 0;
+          left: 0;
+          width: 100%;
+          height: 100%;
+          z-index: -1;
+          pointer-events: none;
+          overflow: hidden;
+        }
+
+        @keyframes particleFloat {
+          from {
+            transform: translateY(0px);
+          }
+          to {
+            transform: translateY(-${spacing}px);
+          }
+        }
+
+        /* Light mode - subtle gray particles */
+        [data-theme="light"] .particle-background {
+          opacity: 0.3;
+        }
+
+        [data-theme="light"] .particle-layer {
+          --particle-color: #6366f1;
+        }
+
+        /* Dark mode - white particles */
+        [data-theme="dark"] .particle-background {
+          opacity: 0.4;
+        }
+
+        [data-theme="dark"] .particle-layer {
+          --particle-color: #ffffff;
+        }
+      `}</style>
+
+      {/* Multiple layers with different speeds for depth effect */}
+      <ParticleLayer count={300} size={1} duration={80} color="var(--particle-color, #fff)" spacing={spacing} />
+      <ParticleLayer count={200} size={2} duration={120} color="var(--particle-color, #fff)" spacing={spacing} />
+      <ParticleLayer count={100} size={2} duration={160} color="var(--particle-color, #fff)" spacing={spacing} />
+    </div>
+  );
+}