refactor(transcription): typed BlockConflictResolvedError instead of prose throw

Felix #3: the 409 path was throwing a human-prose Error which read like
an i18n string that escaped translation. Replace with a named class
carrying code='CONFLICT_RESOLVED' so callers can branch on intent and
future error reporters can map the structured code instead of grepping
strings.

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

frontend/src/lib/utils/blockConflictMerge.spec.ts

@@ -1,5 +1,5 @@
 import { describe, it, expect } from 'vitest';
-import { mergeBlockOnConflict } from './blockConflictMerge';
+import { BlockConflictResolvedError, mergeBlockOnConflict } from './blockConflictMerge';
 import type { PersonMention, TranscriptionBlockData } from '$lib/types';
 
 const baseBlock: TranscriptionBlockData = {
@@ -85,6 +85,21 @@ describe('mergeBlockOnConflict', () => {
 		expect(merged.version).toBe(42);
 	});
 
+	it('carries server-only mention array through when local has none', () => {
+		const merged = mergeBlockOnConflict({
+			serverBlock: {
+				...baseBlock,
+				mentionedPersons: [
+					{ personId: 'p-aug', displayName: 'Augusta Raddatz' },
+					{ personId: 'p-anna', displayName: 'Anna Schmidt' }
+				]
+			},
+			localText: 'x',
+			localMentions: []
+		});
+		expect(merged.mentionedPersons).toHaveLength(2);
+	});
+
 	it('carries other server fields (sortOrder, reviewed, updatedAt) forward', () => {
 		const merged = mergeBlockOnConflict({
 			serverBlock: {
@@ -101,3 +116,13 @@ describe('mergeBlockOnConflict', () => {
 		expect(merged.updatedAt).toBe('2026-04-29T10:00:00Z');
 	});
 });
+
+describe('BlockConflictResolvedError', () => {
+	it('is an Error with code = CONFLICT_RESOLVED', () => {
+		const err = new BlockConflictResolvedError('block-1');
+		expect(err).toBeInstanceOf(Error);
+		expect(err.code).toBe('CONFLICT_RESOLVED');
+		expect(err.name).toBe('BlockConflictResolvedError');
+		expect(err.message).toContain('block-1');
+	});
+});

frontend/src/lib/utils/blockConflictMerge.ts

@@ -1,5 +1,21 @@
 import type { PersonMention, TranscriptionBlockData } from '$lib/types';
 
+/**
+ * Sentinel thrown by saveBlock after a 409 rename-mid-edit has been merged
+ * into local state. Surfaces to the autosave hook as an error (so the UI
+ * shows the retry indicator), but distinguishable from a genuine network
+ * failure via the code.
+ */
+export class BlockConflictResolvedError extends Error {
+	readonly code = 'CONFLICT_RESOLVED' as const;
+	constructor(blockId: string) {
+		super(
+			`Block ${blockId} was rebased onto the latest server snapshot — retry to save the merged result`
+		);
+		this.name = 'BlockConflictResolvedError';
+	}
+}
+
 type MergeArgs = {
 	serverBlock: TranscriptionBlockData;
 	localText: string;

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

@@ -100,7 +100,8 @@ async function saveBlock(
 	});
 	if (res.status === 409) {
 		// Rename-mid-edit (B12b): refetch latest, merge so transcriber input survives.
-		const { mergeBlockOnConflict } = await import('$lib/utils/blockConflictMerge');
+		const { mergeBlockOnConflict, BlockConflictResolvedError } =
+			await import('$lib/utils/blockConflictMerge');
 		const fresh = await fetch(`/api/documents/${doc.id}/transcription-blocks/${blockId}`);
 		if (fresh.ok) {
 			const serverBlock = await fresh.json();
@@ -111,7 +112,7 @@ async function saveBlock(
 			});
 			transcriptionBlocks = transcriptionBlocks.map((b) => (b.id === blockId ? merged : b));
 		}
-		throw new Error('Conflict resolved — please save again');
+		throw new BlockConflictResolvedError(blockId);
 	}
 	if (!res.ok) throw new Error('Save failed');
 	const updated = await res.json();