fix architectural issues: SPA nav, error handling, CSS bugs, perf

- responsive.js: fix scroll race condition (scroll after innerHTML),
  add error handling for fetch failures, fix implicit global `eid`,
  dispatch `beforenavigate` event for cleanup hooks
- chessbed.js: fix implicit global `ratings` variable
- status.js: clear polling interval on SPA navigation via
  `beforenavigate` event to prevent leak
- App.css: add font-display:swap to all @font-face, fix broken
  media query (missing px unit), consolidate duplicate selectors
  (.concentratedHead, .relative, strong), fix hardcoded bookshelf
  background-image path to use relative URL
- header.html: defer chessbed.js, use p5.min.js instead of p5.js
- monitor.py: use ThreadPoolExecutor for concurrent service checks
- config.py: fix __import__('envs.py') → __import__('envs')
- app.py: rename misleading error handlers (page404→handle_http_error,
  page500→handle_generic_error), fix error info leakage by not passing
  raw exception to InternalServerError, fix hardcoded canonical "404"

https://claude.ai/code/session_01FUhPqQLahEoL6FMxhXkDKa

src/static/js/status.js

@@ -248,6 +248,14 @@ function initStatusPage() {
   statusIntervalId = setInterval(fetchStatus, 300000);
 }
 
+// Clean up interval when navigating away via SPA
+document.addEventListener('beforenavigate', () => {
+  if (statusIntervalId !== null) {
+    clearInterval(statusIntervalId);
+    statusIntervalId = null;
+  }
+});
+
 // Start when page loads
 if (document.readyState === 'loading') {
   document.addEventListener('DOMContentLoaded', initStatusPage);