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/components/TapZone.svelte

@@ -1,5 +1,6 @@
 <script lang="ts">
     import { onMount } from 'svelte';
+    import { swipeDirection } from '$lib/swipe';
 
     let {
         onPrev,
@@ -27,6 +28,9 @@
 
     let pressTimer: ReturnType<typeof setTimeout> | null = null;
     let pressStart: { x: number; y: number } | null = null;
+    // Swipe origin — tracked for every touch pointerdown (independent of the
+    // long-press timer) so a horizontal drag can turn the page.
+    let swipeStart: { x: number; y: number } | null = null;
     // Self-expiring suppression: cleared either by an actual click
     // reaching `withSuppress` or by the SUPPRESS_EXPIRY_MS fallback
     // timer below. The latter matters when the user lifts off the
@@ -54,8 +58,11 @@
     }
 
     function onPointerDown(e: PointerEvent) {
-        if (!onLongPress) return;
         if (e.pointerType !== 'touch') return;
+        // Record the swipe origin for every touch, whether or not long-press
+        // is wired up.
+        swipeStart = { x: e.clientX, y: e.clientY };
+        if (!onLongPress) return;
         clearPress();
         pressStart = { x: e.clientX, y: e.clientY };
         const startX = e.clientX;
@@ -79,17 +86,35 @@
         if (Math.hypot(dx, dy) > MOVE_TOLERANCE) clearPress();
     }
 
-    function onPointerUp() {
+    function onPointerUp(e: PointerEvent) {
+        if (swipeStart && e.pointerType === 'touch') {
+            const dir = swipeDirection(e.clientX - swipeStart.x, e.clientY - swipeStart.y);
+            if (dir) {
+                // A swipe consumed the gesture — suppress the click the
+                // browser may synthesize on the zone so it doesn't also fire
+                // that zone's tap action (reusing the long-press suppressor).
+                suppressNextClick = true;
+                if (suppressExpiryTimer != null) clearTimeout(suppressExpiryTimer);
+                suppressExpiryTimer = setTimeout(() => {
+                    suppressNextClick = false;
+                    suppressExpiryTimer = null;
+                }, SUPPRESS_EXPIRY_MS);
+                (dir === 'next' ? onNext : onPrev)();
+            }
+        }
+        swipeStart = null;
         clearPress();
     }
 
     function onPointerCancel() {
+        swipeStart = null;
         clearPress();
     }
 
     function onScroll() {
         // Scrolling while pressed almost always means the user is
         // panning, not deliberately holding. Cancel.
+        swipeStart = null;
         clearPress();
     }