feat(overlay): add optional subtitle selection modal

- Add primary and secondary mpv track selection from the overlay
- Support configurable sequence shortcuts with conflict detection and hot reload
- Document settings, shortcuts, and generated config defaults

src/shared/subtitle-selection.ts

@@ -0,0 +1,33 @@
+export interface SubtitleSelectionState {
+  mediaPath: string;
+  tracks: { id: number; label: string }[];
+  primary: number | null;
+  secondary: number | null;
+}
+
+export type SubtitleSelectionRequest = Pick<
+  SubtitleSelectionState,
+  'mediaPath' | 'primary' | 'secondary'
+>;
+
+export function parseSubtitleSelectionRequest(value: unknown): SubtitleSelectionRequest {
+  if (
+    typeof value !== 'object' ||
+    value === null ||
+    !('mediaPath' in value) ||
+    typeof value.mediaPath !== 'string' ||
+    !value.mediaPath ||
+    !('primary' in value) ||
+    !isTrackSelection(value.primary) ||
+    !('secondary' in value) ||
+    !isTrackSelection(value.secondary)
+  )
+    throw new Error('Invalid subtitle selection.');
+  if (value.primary !== null && value.primary === value.secondary)
+    throw new Error('Choose different primary and secondary tracks.');
+  return { mediaPath: value.mediaPath, primary: value.primary, secondary: value.secondary };
+}
+
+function isTrackSelection(value: unknown): value is number | null {
+  return value === null || (typeof value === 'number' && Number.isSafeInteger(value) && value > 0);
+}