fix: add GridStackLayout showcase view (#7)

Adds a /gridstack route demonstrating GridStackLayout: four default
widgets (line/bar/pie chart cards plus a usage hint), buttons to add and
remove widgets at runtime, a reset-layout button and a layout-change
status line. Layout is persisted per browser via the existing
localStorage support. The view gets its own SideNavItem in MainLayout
and full de/en/es translations.

Fixes an infinite recursion in GridStackLayout.add(Component): a
single-argument add(item) resolves to that overload rather than the
varargs one, so its delegation to add(item) called itself. It now
appends through the element API and passes a GridStackItem through
unwrapped.

Adds the first test sources (browserless UI tests) covering the view's
widget count and the add/remove buttons.

Closes #7

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PYAcKoXKPN3rJ7nBroKZkc

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

@@ -20,7 +20,11 @@ public enum Fa {
     FORM("fa-solid", "fa-rectangle-list"),
     TABLE("fa-solid", "fa-table"),
     GLOBE("fa-solid", "fa-globe"),
-    SEARCH("fa-solid", "fa-magnifying-glass");
+    SEARCH("fa-solid", "fa-magnifying-glass"),
+    GRID("fa-solid", "fa-table-cells-large"),
+    ADD("fa-solid", "fa-plus"),
+    REMOVE("fa-solid", "fa-trash"),
+    RESET("fa-solid", "fa-arrow-rotate-left");
 
     private final String[] classes;