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

docker-entrypoint.sh

@@ -1,5 +1,9 @@
 #!/bin/sh
 
+# Set default port if not provided
+: "${APP_PORT:=80}"
+export APP_PORT
+
 # Generate ./assets/sync-config-default-override.json from environment variables
 JSON="{}"
 JSON_PATH=./assets/sync-config-default-override.json