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

views/index.ejs

@@ -22,6 +22,9 @@
 	<link rel="prefetch" href="fonts/Star4000 Extended.woff" as="font" type="font/woff" crossorigin>
 	<link rel="prefetch" href="fonts/Star4000 Large.woff" as="font" type="font/woff" crossorigin>
 	<link rel="prefetch" href="fonts/Star4000 Small.woff" as="font" type="font/woff" crossorigin>
+	<% if (typeof serverAvailable !== 'undefined' && serverAvailable) { %>
+	<script>window.WS4KP_SERVER_AVAILABLE=true;</script>
+	<% } %>
 
   <% if (production) { %>
   <link rel="stylesheet" type="text/css" href="resources/ws.min.css?_=<%=production%>" />