Fix duplicate const BASE_PATH declaration in chat.js

main.js and chat.js both declared const BASE_PATH at top level.
When both load on index.html, the second const throws SyntaxError,
preventing chat.js from executing (chat FAB button unresponsive).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/static/js/chat.js

@@ -1,5 +1,6 @@
 // ── Chat Widget JavaScript ──
-const BASE_PATH = window.BASE_PATH || '';
+// BASE_PATH is declared in main.js (loaded first on index.html)
+if (typeof BASE_PATH === 'undefined') var BASE_PATH = window.BASE_PATH || '';
 
 let chatSessionId = sessionStorage.getItem('chat_session');
 if (!chatSessionId) {