refactor(transcribe): extract region navigation into a tested pure helper (#327)

Review follow-up (Sara): j/k wrap-around and fresh-entry had no direct
coverage — the logic lived inline in the page where the action spec only
mocks the callbacks. Extracted to a pure stepRegion() with 9 unit tests
(empty list, forward/back, both wraps, fresh-entry null + unknown id,
length-1). Also replaces the inline nested ternary Felix flagged.

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

frontend/src/lib/document/transcription/regionNavigation.spec.ts

@@ -0,0 +1,48 @@
+import { describe, it, expect } from 'vitest';
+import { stepRegion } from './regionNavigation';
+
+describe('stepRegion', () => {
+	const ids = ['a', 'b', 'c'];
+
+	it('returns null for an empty list', () => {
+		expect(stepRegion([], null, 1)).toBe(null);
+		expect(stepRegion([], 'a', -1)).toBe(null);
+	});
+
+	it('steps forward from the middle', () => {
+		expect(stepRegion(ids, 'a', 1)).toBe('b');
+		expect(stepRegion(ids, 'b', 1)).toBe('c');
+	});
+
+	it('steps backward from the middle', () => {
+		expect(stepRegion(ids, 'c', -1)).toBe('b');
+		expect(stepRegion(ids, 'b', -1)).toBe('a');
+	});
+
+	it('wraps forward past the last region to the first', () => {
+		expect(stepRegion(ids, 'c', 1)).toBe('a');
+	});
+
+	it('wraps backward past the first region to the last', () => {
+		expect(stepRegion(ids, 'a', -1)).toBe('c');
+	});
+
+	it('lands on the first region when entering fresh (no active) going forward', () => {
+		expect(stepRegion(ids, null, 1)).toBe('a');
+	});
+
+	it('lands on the last region when entering fresh (no active) going backward', () => {
+		expect(stepRegion(ids, null, -1)).toBe('c');
+	});
+
+	it('treats an unknown active id as a fresh entry', () => {
+		expect(stepRegion(ids, 'zzz', 1)).toBe('a');
+		expect(stepRegion(ids, 'zzz', -1)).toBe('c');
+	});
+
+	it('returns the single region for both directions (wrap of length 1)', () => {
+		expect(stepRegion(['only'], 'only', 1)).toBe('only');
+		expect(stepRegion(['only'], 'only', -1)).toBe('only');
+		expect(stepRegion(['only'], null, 1)).toBe('only');
+	});
+});

frontend/src/lib/document/transcription/regionNavigation.ts

@@ -0,0 +1,33 @@
+/**
+ * Region navigation for the transcribe keyboard shortcuts (j/k) — issue #327.
+ *
+ * Pure and side-effect free so the wrap-around / fresh-entry branches are
+ * unit-testable without mounting the page.
+ */
+
+/**
+ * Pick the annotation id one step from the active region, wrapping around the
+ * ends. Entering fresh (no active region, or an unknown id) lands on the first
+ * region going forward and the last going backward.
+ *
+ * @param orderedAnnotationIds region annotation ids in display order
+ * @param activeId the currently active region, or null
+ * @param delta +1 for next (j), -1 for previous (k)
+ * @returns the next annotation id, or null when there are no regions
+ */
+export function stepRegion(
+	orderedAnnotationIds: string[],
+	activeId: string | null,
+	delta: 1 | -1
+): string | null {
+	const count = orderedAnnotationIds.length;
+	if (count === 0) return null;
+
+	const current = activeId === null ? -1 : orderedAnnotationIds.indexOf(activeId);
+	if (current === -1) {
+		return delta > 0 ? orderedAnnotationIds[0] : orderedAnnotationIds[count - 1];
+	}
+
+	const next = (current + delta + count) % count;
+	return orderedAnnotationIds[next];
+}