fix: make dashboard widgets closable (#14)

Every GridStackItem now renders a close button next to its drag grip,
mirroring the grip's hover affordance. Closing detaches the item
server-side; grid-stack.ts already unregisters widgets via its
MutationObserver, so the layout is persisted without an extra protocol.

Closable is on by default and can be turned off per item with
setClosable(false); GridStackItem.CloseEvent lets views react.

The GridStackView "remove last" toolbar button is dropped — per-widget
close replaces it, so the view no longer tracks a widget stack. Its
counter now only grows, keeping a closed widget's id from being handed
to a new widget (which would inherit the saved position).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ue9ZtWUBQF4SuSHpzZ3zwq

src/main/java/com/example/components/Fa.java

@@ -25,7 +25,8 @@ public enum Fa {
     ADD("fa-solid", "fa-plus"),
     REMOVE("fa-solid", "fa-trash"),
     RESET("fa-solid", "fa-arrow-rotate-left"),
-    DRAG("fa-solid", "fa-grip-vertical");
+    DRAG("fa-solid", "fa-grip-vertical"),
+    CLOSE("fa-solid", "fa-xmark");
 
     private final String[] classes;