fix: update deployment guide with critical URL configuration and nginx port fixes

- Add prominent warning about FRONTEND_URL configuration requiring exact port match
- Add comprehensive troubleshooting section for 502/CORS login failures
- Fix nginx.conf to use correct backend port (3001 instead of 3000)
- Document common deployment issues and their solutions
- Explain Docker DNS caching issues after container restarts

🤖 Generated with [Claude Code](https://claude.ai/code)

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

frontend/nginx.conf

@@ -39,7 +39,7 @@ server {
 
     # API proxy
     location /api {
-        proxy_pass http://backend:3000;
+        proxy_pass http://backend:3001;
         proxy_http_version 1.1;
         proxy_set_header Upgrade $http_upgrade;
         proxy_set_header Connection 'upgrade';
@@ -53,7 +53,7 @@ server {
 
     # Photo serving proxy
     location /photos {
-        proxy_pass http://backend:3000;
+        proxy_pass http://backend:3001;
         proxy_http_version 1.1;
         proxy_set_header Host $host;
         proxy_set_header X-Real-IP $remote_addr;
@@ -67,7 +67,7 @@ server {
 
     # Thumbnail serving proxy
     location /thumbnails {
-        proxy_pass http://backend:3000;
+        proxy_pass http://backend:3001;
         proxy_http_version 1.1;
         proxy_set_header Host $host;
         proxy_set_header X-Real-IP $remote_addr;
@@ -81,7 +81,7 @@ server {
 
     # Uploads serving proxy (logos, favicons, watermarks)
     location /uploads {
-        proxy_pass http://backend:3000;
+        proxy_pass http://backend:3001;
         proxy_http_version 1.1;
         proxy_set_header Host $host;
         proxy_set_header X-Real-IP $remote_addr;