feat(ui): add trapFocus action for modal dialogs (T7.5, FR-UX-4)

Moves focus into the dialog on mount, cycles Tab/Shift+Tab within its
focusable elements, and restores focus to whatever had it before the
dialog opened once it closes.

src/lib/actions/trapFocus.ts

@@ -0,0 +1,40 @@
+// Svelte action for modal dialogs (T7.5, FR-UX-4): moves focus into the node
+// on mount, cycles Tab/Shift+Tab within its focusable elements, and restores
+// focus to whatever had it before the dialog opened once it closes.
+
+const FOCUSABLE =
+  'a[href], button:not([disabled]), input:not([disabled]), select:not([disabled]), textarea:not([disabled]), [tabindex]:not([tabindex="-1"])';
+
+export function trapFocus(node: HTMLElement) {
+  const previouslyFocused = document.activeElement as HTMLElement | null;
+
+  function focusables(): HTMLElement[] {
+    return Array.from(node.querySelectorAll<HTMLElement>(FOCUSABLE));
+  }
+
+  (focusables()[0] ?? node).focus();
+
+  function onKeydown(e: KeyboardEvent) {
+    if (e.key !== "Tab") return;
+    const items = focusables();
+    if (items.length === 0) return;
+    const first = items[0];
+    const last = items[items.length - 1];
+    if (e.shiftKey && document.activeElement === first) {
+      e.preventDefault();
+      last.focus();
+    } else if (!e.shiftKey && document.activeElement === last) {
+      e.preventDefault();
+      first.focus();
+    }
+  }
+
+  node.addEventListener("keydown", onKeydown);
+
+  return {
+    destroy() {
+      node.removeEventListener("keydown", onKeydown);
+      previouslyFocused?.focus();
+    },
+  };
+}