feat(reader): swipe left/right to turn pages in single mode

Wire horizontal swipe into the mobile tap-zone overlay (single mode only,
so continuous mode keeps native vertical scroll). A leftward swipe past the
threshold turns to the next page, rightward to the previous; a mostly-
vertical drag is ignored so panning a tall page never turns it. The gesture
suppresses the synthesized zone tap so it doesn't double-fire. Swipe
classification is a pure, unit-tested helper; e2e drives real touch pointer
events for horizontal (both directions) and vertical cases.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/lib/swipe.ts

@@ -0,0 +1,23 @@
+// Pure gesture helper for the reader's single-page swipe navigation. Kept
+// UI-free so the "is this a page-turn swipe, and which way" decision is
+// unit-testable without dispatching pointer events.
+
+export const SWIPE_THRESHOLD_PX = 45;
+
+/**
+ * Classifies a pointer displacement as a page-turn swipe. Returns `'next'`
+ * for a leftward swipe, `'prev'` for a rightward one, or `null` when the
+ * gesture is too short or too vertical to be a deliberate horizontal swipe
+ * (so vertical panning never turns the page).
+ */
+export function swipeDirection(
+    dx: number,
+    dy: number,
+    threshold: number = SWIPE_THRESHOLD_PX
+): 'next' | 'prev' | null {
+    if (Math.abs(dx) < threshold) return null;
+    // Must be predominantly horizontal — a diagonal or vertical drag is not
+    // a page turn.
+    if (Math.abs(dx) <= Math.abs(dy)) return null;
+    return dx < 0 ? 'next' : 'prev';
+}