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/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(() => {