feat(transcription): announce re-edit context via the existing live region (#628)

Passes editingDisplayName into MentionDropdown; the persistent aria-live region
announces person_mention_editing_announce({displayName}) on re-edit open and
falls back to the prompt/empty/count copy once the user edits or results arrive.
Routed through the SAME sr-only region as the result count — no second live
region (avoids the double-announce bug Leonie S-2 fixed). Fresh-@ passes an
empty editingDisplayName, so its announcements are unchanged.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/shared/discussion/MentionDropdown.svelte

@@ -33,7 +33,8 @@ let {
 	editorQuery = '',
 	onSearch = () => {},
 	ondismiss = () => {},
-	focusOnMount = false
+	focusOnMount = false,
+	editingDisplayName = ''
 }: {
 	model: DropdownState;
 	/** Text typed after `@` in the host editor. Mirrors into the search input
@@ -46,6 +47,10 @@ let {
 	/** Re-edit (#628) opens with the search field focused; the fresh-@ path keeps
 	 *  focus in the editor so typing flows to the contenteditable. */
 	focusOnMount?: boolean;
+	/** When set (re-edit, #628), the persistent live region announces the editing
+	 *  context for this mention. Routed through the SAME aria-live region as the
+	 *  result count — never a second live region (avoids the double-announce bug). */
+	editingDisplayName?: string;
 } = $props();
 
 let searchQuery = $state(untrack(() => editorQuery.slice(0, MAX_QUERY_LENGTH)));
@@ -260,7 +265,11 @@ function selectItem(item: Person) {
 	-->
 	<p class="sr-only" aria-live="polite">
 		{#if model.items.length === 0}
+			{#if editingDisplayName && !userHasEdited}
+				{m.person_mention_editing_announce({ displayName: editingDisplayName })}
+			{:else}
 				{isQueryEmpty ? m.person_mention_search_prompt() : m.person_mention_popup_empty()}
+			{/if}
 		{:else if model.items.length === 1}
 			{m.person_mention_results_count_singular()}
 		{:else}

frontend/src/lib/shared/discussion/PersonMentionEditor.svelte

@@ -82,7 +82,7 @@ type LooseRenderProps = {
 // and (#628) the pencil re-edit affordance — so at most one dropdown is ever
 // mounted (the AC-6 single-dropdown invariant). open() closes any prior
 // dropdown first; render() is a thin adapter over open()/update()/close().
-type OpenOptions = { focusOnMount?: boolean };
+type OpenOptions = { focusOnMount?: boolean; editingDisplayName?: string };
 
 type MentionController = {
 	open: (clientRect: RectGetter, query: string, commit: CommitFn, opts?: OpenOptions) => void;
@@ -183,6 +183,7 @@ function createMentionController(): MentionController {
 				model: dropdownState,
 				ondismiss: dismiss,
 				focusOnMount: opts.focusOnMount ?? false,
+				editingDisplayName: opts.editingDisplayName ?? '',
 				// MentionDropdown reads `editorQuery` off the shared state proxy via
 				// this getter — Svelte 5's mount() does not expose settable prop
 				// accessors, so we route through the proxy (same pattern as items).
@@ -235,7 +236,10 @@ function commitRelink(pos: number): CommitFn {
 // the stored displayName.
 function requestRelink(getRect: () => DOMRect | null, displayName: string, pos: number) {
 	if (!editor || !editor.isEditable) return;
-	controller.open(getRect, displayName, commitRelink(pos), { focusOnMount: true });
+	controller.open(getRect, displayName, commitRelink(pos), {
+		focusOnMount: true,
+		editingDisplayName: displayName
+	});
 }
 
 onMount(() => {

frontend/src/lib/shared/discussion/PersonMentionEditor.svelte.spec.ts

@@ -1309,3 +1309,27 @@ describe('PersonMentionEditor — #628 security', () => {
 		});
 	});
 });
+
+// ─── #628 NFR a11y: editing context announced via the existing live region ───
+
+describe('PersonMentionEditor — #628 editing announce', () => {
+	it('re-edit open announces the editing context through the single persistent live region', async () => {
+		mockFetchEmpty();
+		renderHost({ value: '@Aug ', mentionedPersons: [{ personId: 'p-aug', displayName: 'Aug' }] });
+		await vi.waitFor(() =>
+			expect(document.querySelector('[data-type="mention"] button')).not.toBeNull()
+		);
+		(document.querySelector('[data-type="mention"] button') as HTMLElement).dispatchEvent(
+			new MouseEvent('click', { bubbles: true, cancelable: true })
+		);
+
+		await vi.waitFor(() => {
+			const liveRegions = document.querySelectorAll('[role="listbox"] [aria-live="polite"]');
+			// Exactly ONE live region (no second announcer — avoids double-announce).
+			expect(liveRegions.length).toBe(1);
+			expect(liveRegions[0].textContent ?? '').toContain(
+				m.person_mention_editing_announce({ displayName: 'Aug' })
+			);
+		});
+	});
+});