fix: prefer direction with more available space for dropdown

Compare space above vs below the trigger and open toward whichever
side has more room. Prevents flipping upward when the menu would
overflow even more in that direction.

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

src/renderer/extensions/vueNodes/widgets/components/form/dropdown/FormDropdown.vue

@@ -146,7 +146,9 @@ function toggleDropdown() {
   if (disabled) return
   if (!isOpen.value && triggerRef.value) {
     const rect = triggerRef.value.getBoundingClientRect()
-    openUpward.value = rect.bottom + MENU_HEIGHT > window.innerHeight
+    const spaceBelow = window.innerHeight - rect.bottom
+    const spaceAbove = rect.top
+    openUpward.value = spaceBelow < MENU_HEIGHT && spaceAbove > spaceBelow
   }
   isOpen.value = !isOpen.value
 }