feat: modernize login page with dark theme and breathing logo animation

- Dark gradient background (slate-950/blue-950) with ambient blur effects
- Circular logo centered with dual-ring frosted glass design
- Heartbeat breathing animation (3s cycle) with glow pulse on outer ring
- Gradient sign-in button with hover shadow effects
- Removed "first user" warning, replaced with subtle "authorized personnel" note
- Closes #5 and #6

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

frontend/tailwind.config.js

@@ -66,12 +66,25 @@ export default {
       },
       animation: {
         'bounce-subtle': 'bounce-subtle 2s ease-in-out infinite',
+        'heartbeat': 'heartbeat 3s ease-in-out infinite',
+        'glow-pulse': 'glow-pulse 3s ease-in-out infinite',
       },
       keyframes: {
         'bounce-subtle': {
           '0%, 100%': { transform: 'translateY(0)' },
           '50%': { transform: 'translateY(-2px)' },
         },
+        'heartbeat': {
+          '0%, 100%': { transform: 'scale(1)' },
+          '15%': { transform: 'scale(1.08)' },
+          '30%': { transform: 'scale(1)' },
+          '45%': { transform: 'scale(1.05)' },
+          '60%': { transform: 'scale(1)' },
+        },
+        'glow-pulse': {
+          '0%, 100%': { boxShadow: '0 0 20px rgba(59, 130, 246, 0.15), 0 0 60px rgba(59, 130, 246, 0.05)' },
+          '50%': { boxShadow: '0 0 30px rgba(59, 130, 246, 0.3), 0 0 80px rgba(59, 130, 246, 0.1)' },
+        },
       },
     },
   },