refactor(e2e): migrate to production Dockerfile for E2E tests

Complete E2E Docker migration (Steps 1-3):

1. Add SPA routing support in nginx (try_files directive)
2. Make APP_PORT configurable via environment variable
3. Migrate docker-compose.e2e.yaml from dev server to production build

Changes:
- docker-compose.e2e.yaml: Use production Dockerfile instead of dev server
  - Remove volume mounts (self-contained production build)
  - Add UNSPLASH build args
  - Add WEBDAV_BACKEND environment variable
  - Reduce healthcheck start_period from 120s to 30s (nginx is faster)
- nginx/default.conf.template: Add try_files for SPA routing, use APP_PORT
- docker-entrypoint.sh: Export APP_PORT with default value

Benefits:
- Production build provides more realistic test environment
- Faster startup (30s vs 120s)
- No dependency on local node_modules
- Matches production deployment more closely

nginx/default.conf.template

@@ -4,13 +4,14 @@ map "" $webdav_backend {
 }
 
 server {
-    listen       80;
+    listen       ${APP_PORT};
     server_name  localhost;
 
     # serve super-productivity as static files at the path /
     location / {
         root   /usr/share/nginx/html;
         index  index.html index.htm;
+        try_files $uri $uri/ /index.html;
     }
 
     # forward requests starting with "/webdav/" to $WEBDAV_BACKEND