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}
-			{isQueryEmpty ? m.person_mention_search_prompt() : m.person_mention_popup_empty()}
+			{#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}