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

gulp/publish-frontend.mjs

@@ -141,6 +141,7 @@ const compressHtml = async () => {
 	return src(htmlSources)
 		.pipe(ejs({
 			production: version,
+			serverAvailable: false,
 			version,
 			OVERRIDES,
 			query: {},