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

Dockerfile.server

@@ -0,0 +1,13 @@
+FROM node:24-alpine
+WORKDIR /app
+
+COPY package.json package-lock.json ./
+RUN npm ci --omit=dev --legacy-peer-deps
+COPY . .
+
+RUN npm run build
+
+EXPOSE 8080
+
+ENV DIST=1
+CMD ["npm", "start"]