feat: add design system with colors, components, and theme configuration

- Introduced a comprehensive color palette in colors.ts, including primary, accent, success, warning, error, info, neutral, slate, and semantic colors for light and dark modes.
- Created a components-registry.ts to manage UI components with metadata, including buttons, inputs, cards, layout, feedback, and navigation components.
- Developed a theme.ts file to centralize typography, spacing, border radius, shadows, z-index, transitions, breakpoints, containers, and responsive utilities.
- Configured Nginx for development with a new nginx.dev.conf, routing API requests to the Flask backend and frontend requests to the Vite development server.

nginx/nginx.dev.conf

@@ -0,0 +1,40 @@
+upstream frontend {
+    server frontend:5173;
+}
+
+# ── HTTP Development Server ──
+server {
+    listen 80 default_server;
+    client_max_body_size 100M;
+    resolver 127.0.0.11 valid=30s ipv6=off;
+    set $backend_upstream backend:5000;
+
+    # API requests → Flask backend
+    location /api/ {
+        proxy_pass http://$backend_upstream;
+        proxy_set_header Host $host;
+        proxy_set_header X-Real-IP $remote_addr;
+        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+        proxy_set_header X-Forwarded-Proto $scheme;
+
+        # Timeout for large file uploads
+        proxy_read_timeout 300s;
+        proxy_send_timeout 300s;
+        proxy_connect_timeout 60s;
+    }
+
+    # Frontend (Vite dev server)
+    location / {
+        proxy_pass http://frontend;
+        proxy_set_header Host $host;
+        proxy_set_header X-Real-IP $remote_addr;
+        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+        proxy_set_header Upgrade $http_upgrade;
+        proxy_set_header Connection "upgrade";
+    }
+
+    # Health check
+    location /health {
+        proxy_pass http://$backend_upstream/api/health;
+    }
+}