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/monitor.py

@@ -5,6 +5,7 @@ Checks service availability and tracks uptime statistics
 import requests
 import time
 import json
+from concurrent.futures import ThreadPoolExecutor
 from datetime import datetime, timedelta
 from threading import Thread, Lock
 from pathlib import Path
@@ -125,12 +126,15 @@ class ServiceMonitor:
         """Check all services and update status data"""
         print(f"[{datetime.now().strftime('%Y-%m-%d %H:%M:%S')}] Checking all services...")
 
-        # Perform all network checks OUTSIDE the lock to avoid blocking API calls
+        # Perform all network checks concurrently and OUTSIDE the lock
         results = {}
-        for service in SERVICES:
-            result = self.check_service(service)
-            results[service['id']] = result
-            print(f"  {service['name']}: {result['status']} ({result['response_time']}ms)")
+        with ThreadPoolExecutor(max_workers=len(SERVICES)) as executor:
+            futures = {executor.submit(self.check_service, s): s for s in SERVICES}
+            for future in futures:
+                service = futures[future]
+                result = future.result()
+                results[service['id']] = result
+                print(f"  {service['name']}: {result['status']} ({result['response_time']}ms)")
 
         # Only acquire lock when updating the shared data structure
         with self.lock: