Add STATIC environment variable for browser-only deployment mode

Implement STATIC=1 environment variable to enable browser-only deployment
without proxy server infrastructure. Uses WS4KP_SERVER_AVAILABLE flag to
distinguish between server-backed and static deployments for proper URL
rewriting and User-Agent header handling.

- Add STATIC env var to skip proxy route registration at startup
- Inject WS4KP_SERVER_AVAILABLE flag via EJS template based on STATIC mode
- Update fetch.mjs to conditionally send User-Agent headers based on server availability
- Update url-rewrite.mjs to skip proxy rewriting when server is unavailable
- Use renderIndex helper for consistent template data across dev/prod modes
- Improve music playlist logging

Benefits of integrated approach:
- Single environment variable controls both server and client behavior
- Flag injection happens once at render time, not on every request
- No runtime HTML string manipulation overhead
- Clean separation between server-backed and static deployment logic
- Same codebase supports both deployment modes without duplication

Static mode (STATIC=1): Direct API calls to external services, no caching
Server mode (default): Local proxy with caching and API request observability

README.md

@@ -48,11 +48,20 @@ npm i
 node index.mjs
 ```
 
-To run via Docker: 
-```
+To run via Docker using a "static deployment" where everything happens in the browser (no server component):
+
+```bash
 docker run -p 8080:8080 ghcr.io/netbymatt/ws4kp
 ```
-Open your web browser: http://localhost:8080/ 
+
+To run via Docker using a "server deployment" with a caching proxy server for multi-client performance and enhanced observability (the same as `npm start`):
+
+```bash
+docker build -f Dockerfile.server -t ws4kp-server .
+docker run -p 8080:8080 ws4kp-server
+```
+
+Open your web browser: http://localhost:8080/
 
 To run via Docker Compose (docker-compose.yaml):
 ```