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/templates/header.html

@@ -53,9 +53,9 @@
     <link rel="canonical" href="{{ var['canonical'] }}" />
     <script defer src="{{ url_for('static', filename='js/checkbox.js') }}"></script>
     <script defer src="{{ url_for('static', filename='js/responsive.js') }}"></script>
-    <script src="{{ url_for('static', filename='js/chessbed.js') }}"></script>
+    <script defer src="{{ url_for('static', filename='js/chessbed.js') }}"></script>
     <script defer src="{{ url_for('static', filename='js/idler.js') }}"></script>
-    <script defer src="https://cdn.jsdelivr.net/npm/p5@1.4.1/lib/p5.js"></script>
+    <script defer src="https://cdn.jsdelivr.net/npm/p5@1.4.1/lib/p5.min.js"></script>
     <title>{{ var['title'] }}</title>
   </head>
   {% block header %}