fix(a11y): give every wa-dialog an accessible name

Eleven dialogs passed a `label` that never reached the accessibility
tree: Web Awesome renders it into an <h2 id="title"> in the same shadow
root as the native <dialog> and never points aria-labelledby at it, so
getByRole('dialog', {name}) matched nothing and a screen reader
announced an unnamed dialog. a11y.md lists all of them under "what is
already correct".

utils/name-dialog.ts sets the IDREF, with aria-label as the fallback for
without-header (first-run-wizard), called from each host's updated().
aria-labelledby rather than aria-label because three call sites compute
their label at render time, and the heading re-renders anyway. It waits
for the dialog's own first update: wa-dialog populates its shadow root
in its own update, so a query at the host's firstUpdated names nothing.

Reaching into another library's open shadow root is deliberate and the
failure is bounded — if the structure moves, the query misses and the
dialog is as unnamed as it was.

frontend/src/components/first-run-wizard/first-run-wizard.ts

@@ -8,6 +8,7 @@ import {
 } from '@go/library/Library';
 import { DirectoryPicker } from '@go/frontendutil/FrontendUtil';
 import { describeError, explainError } from '@utils/describe-error';
+import { nameDialogsIn } from '@utils/name-dialog';
 
 /**
  * First-run setup wizard.
@@ -164,6 +165,15 @@ export class FirstRunWizard extends LitElement {
         }
     `;
 
+    /**
+     * Web Awesome renders `label` into a heading it never points the
+     * `<dialog>` at, so the dialog has no accessible name until
+     * something sets one. See `utils/name-dialog.ts`.
+     */
+    override updated() {
+        nameDialogsIn(this.shadowRoot);
+    }
+
     override render() {
         if (!this.active) return nothing;