Modals: emit lt:modalclose so openers can react to any dismissal

A modal can be dismissed four ways: the ✕ button, a Cancel button, a backdrop
click, or Escape. The last two are handled globally in base.js, so code that
wires only the buttons never learns the modal went away — leaving optimistic UI
in place, and leaving dynamically-inserted overlays in the DOM (where the next
open inserts a duplicate id that shadows the live one).

closeModal now dispatches a bubbling lt:modalclose CustomEvent on the overlay.
Existing button handlers keep working; openers that care can listen for it.

base.js

@@ -241,6 +241,11 @@
         trigger.focus();
       }
     }
+    // Announce the close so whoever opened the modal can undo optimistic UI or
+    // clean up a dynamically-inserted overlay. A modal can be dismissed four
+    // ways — the ✕ button, a Cancel button, a backdrop click, and Escape — and
+    // the last two are handled globally here, so button-only listeners miss them.
+    el.dispatchEvent(new CustomEvent('lt:modalclose', { bubbles: true }));
   }
 
   function closeAllModals() {