refactor(transcribe): extract t-mark + draw-cue policy into tested helpers (#327)

Review follow-up (Sara, fast-follow): the t no-active-region guard and the
draw-cue arm/disarm rule lived inline in the page with no direct coverage.
Extracted to pure resolveTrainingMark() (no-op when no region; recognition
enrolled flip) and canArmDraw()/shouldDisarmDraw(), each with unit tests
(10 cases total). The page now arms the draw cue only via canArmDraw and
disarms via shouldDisarmDraw, and routes t through resolveTrainingMark.

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

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

@@ -0,0 +1,14 @@
+import { describe, it, expect } from 'vitest';
+import { canArmDraw, shouldDisarmDraw } from './drawCue';
+
+describe('draw cue policy', () => {
+	it('arms only in edit mode', () => {
+		expect(canArmDraw('edit')).toBe(true);
+		expect(canArmDraw('read')).toBe(false);
+	});
+
+	it('disarms in every mode except edit', () => {
+		expect(shouldDisarmDraw('read')).toBe(true);
+		expect(shouldDisarmDraw('edit')).toBe(false);
+	});
+});