Fix dashboard crash when board list is null

The /boards endpoint can return null instead of an empty array.
Guard with fallback to empty array before calling .map().

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: bricef

internal/http/dashboard/index.html

@@ -253,6 +253,8 @@ <h1>TaskFlow Dashboard</h1>
 }
 
 async function loadDashboard(boards) {
+  boards = boards || [];
+
   // Try to load admin stats — gracefully degrade if not admin.
   let stats = null;
   try {