feat(transcription): handle 409 rename-mid-edit conflict on block save (B12b)

When PersonService renames a person while a transcriber is editing a
block that mentions them, the block-save endpoint returns 409 (carrying
the new ErrorCode.PERSON_RENAME_CONFLICT from PR-A). saveBlock now:

1. Refetches the latest server snapshot of the block.
2. Calls mergeBlockOnConflict to combine: server's mentionedPersons
   (post-rename displayNames win) + transcriber's unsaved text + any
   local-only mentions added since the last save.
3. Updates the local block state with the merged result.
4. Re-throws so the autosave indicator surfaces the conflict and the
   pending payload is preserved for retry (B12).

The merge logic is a pure function so it can be unit-tested in
isolation and reused for any future conflict-resolution scenarios.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/documents/[id]/+page.svelte

@@ -98,6 +98,21 @@ async function saveBlock(
 		headers: { 'Content-Type': 'application/json' },
 		body: JSON.stringify({ text, mentionedPersons })
 	});
+	if (res.status === 409) {
+		// Rename-mid-edit (B12b): refetch latest, merge so transcriber input survives.
+		const { mergeBlockOnConflict } = await import('$lib/utils/blockConflictMerge');
+		const fresh = await fetch(`/api/documents/${doc.id}/transcription-blocks/${blockId}`);
+		if (fresh.ok) {
+			const serverBlock = await fresh.json();
+			const merged = mergeBlockOnConflict({
+				serverBlock,
+				localText: text,
+				localMentions: mentionedPersons
+			});
+			transcriptionBlocks = transcriptionBlocks.map((b) => (b.id === blockId ? merged : b));
+		}
+		throw new Error('Conflict resolved — please save again');
+	}
 	if (!res.ok) throw new Error('Save failed');
 	const updated = await res.json();
 	transcriptionBlocks = transcriptionBlocks.map((b) => (b.id === blockId ? updated : b));