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.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];
+}