refactor(reader): introduce priority-based touch interceptor for gesture handling (#3809)

Replace the inline touch-swipe event dispatching with a module-level
interceptor registry. This lets the reading ruler (priority 10) claim
drag gestures before the swipe-to-flip handler (priority 0), preventing
conflicts when dragging the ruler on touch devices.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/readest-app/src/app/reader/hooks/useTouchInterceptor.ts

@@ -0,0 +1,58 @@
+import { useEffect, useRef } from 'react';
+
+export interface TouchDetail {
+  phase: 'start' | 'move' | 'end';
+  touch: { screenX: number; screenY: number };
+  touchStart: { screenX: number; screenY: number };
+  deltaX: number;
+  deltaY: number;
+  deltaT: number;
+}
+
+export type TouchInterceptorFn = (bookKey: string, detail: TouchDetail) => boolean;
+
+interface TouchInterceptorEntry {
+  handler: TouchInterceptorFn;
+  priority: number;
+}
+
+// Module-level registry — interceptors are called in descending priority order.
+// The first interceptor that returns true consumes the gesture.
+const interceptors = new Map<string, TouchInterceptorEntry>();
+
+export const registerTouchInterceptor = (
+  id: string,
+  handler: TouchInterceptorFn,
+  priority = 0,
+): (() => void) => {
+  interceptors.set(id, { handler, priority });
+  return () => {
+    interceptors.delete(id);
+  };
+};
+
+export const dispatchTouchInterceptors = (bookKey: string, detail: TouchDetail): boolean => {
+  const sorted = [...interceptors.values()].sort((a, b) => b.priority - a.priority);
+  for (const { handler } of sorted) {
+    if (handler(bookKey, detail)) return true;
+  }
+  return false;
+};
+
+/**
+ * React hook for registering a touch interceptor with automatic cleanup.
+ * The handler ref is updated on every render so the interceptor always
+ * calls the latest closure without re-registering.
+ */
+export const useTouchInterceptor = (id: string, handler: TouchInterceptorFn, priority = 0) => {
+  const handlerRef = useRef(handler);
+  handlerRef.current = handler;
+
+  useEffect(() => {
+    return registerTouchInterceptor(
+      id,
+      (bookKey, detail) => handlerRef.current(bookKey, detail),
+      priority,
+    );
+  }, [id, priority]);
+};