fix: valid config.json + content-hashed bundle (cache-bust)

- BuildConfig.sh: ${DEBUG:false} -> ${DEBUG:-false} and normalize to
  true/false. The old syntax produced empty -> invalid config.json
  ("DEBUG": }) when DEBUG was unset, breaking the whole frontend.
- webpack: output [name].[contenthash].js so index.html references a
  unique bundle URL each build; eliminates stale CDN/browser bundle
  after deploys (no manual cache purge needed).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

BuildConfig.sh

@@ -5,7 +5,12 @@ FRONTEND_HOST="${FRONTEND_HOST:-http://localhost:80}"
 KC_CLIENT_ID="${KC_CLIENT_ID:-labdev}"
 KC_HOST="${KC_HOST:-https://login.hangman-lab.top}"
 KC_REALM="${KC_REALM:-Hangman-Lab}"
-DEBUG="${DEBUG:false}"
+# Note: ${DEBUG:-false} (correct default syntax). The old ${DEBUG:false}
+# produced an empty value when DEBUG was unset -> invalid config.json.
+DEBUG="${DEBUG:-false}"
+# DEBUG is emitted unquoted as a JSON boolean — guarantee it is exactly
+# true/false so config.json can never be invalid JSON.
+case "$DEBUG" in true|false) ;; *) DEBUG=false ;; esac
 rm -f /usr/share/nginx/html/config.js