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/download-picker/download-picker.ts

@@ -10,6 +10,7 @@ import { downloadStore } from '@store/download-store';
 import type { download } from '@go/models';
 import './candidate-row';
 import { explainError } from '@utils/describe-error';
+import { nameDialogsIn } from '@utils/name-dialog';
 
 /**
  * The "find this album" dialog: searches every enabled download client,
@@ -119,6 +120,11 @@ export class DownloadPicker extends LitElement {
     ];
 
     override updated(changed: Map<string, unknown>) {
+        // 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`.
+        nameDialogsIn(this.shadowRoot);
+
         if (changed.has('open') && this.open) {
             void this.search();
         }