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.ts

@@ -0,0 +1,20 @@
+/**
+ * Policy for the "draw a new region" keyboard cue (the `n` shortcut) in the
+ * transcribe panel — issue #327.
+ *
+ * The cue is only valid while editing: `n` arms it in edit mode, and it must
+ * clear when a region is drawn or when the panel leaves edit mode. Pure so both
+ * rules are testable without mounting the page.
+ */
+
+type PanelMode = 'read' | 'edit';
+
+/** The draw cue may only be armed while in edit mode. */
+export function canArmDraw(panelMode: PanelMode): boolean {
+	return panelMode === 'edit';
+}
+
+/** Leaving edit mode must disarm the draw cue. */
+export function shouldDisarmDraw(panelMode: PanelMode): boolean {
+	return !canArmDraw(panelMode);
+}