Add double-tap left/right navigation on chapter content page

scraper/internal/server/ui.go

@@ -2104,6 +2104,7 @@ const chapterTmpl = `
 <script>
 (function () {
   var NEXT_N    = {{.NextN}};
+  var PREV_N    = {{.PrevN}};
   var SLUG      = '{{.Slug}}';
   var CHAPTER_N = {{.ChapterN}};
 
@@ -2561,6 +2562,56 @@ const chapterTmpl = `
   // ── initialise queue bar on page load ────────────────────────────────────────
   queueSetCurrent('idle', 'ready — ch.' + CHAPTER_N);
   if (NEXT_N) { queueSetNext('idle', 'waiting — ch.' + NEXT_N); }
+
+  // ── double-tap left/right to navigate chapters ───────────────────────────────
+  // A double-tap on the left third of the screen goes to the previous chapter;
+  // a double-tap on the right third goes to the next chapter.
+  // Taps on interactive elements (buttons, links, inputs, selects) are ignored
+  // so normal UI interactions are never swallowed.
+  (function initDoubleTap() {
+    var lastTap  = 0;
+    var lastSide = '';   // 'left' | 'right'
+    var THRESHOLD = 300; // ms between taps
+
+    var INTERACTIVE = ['A', 'BUTTON', 'INPUT', 'SELECT', 'TEXTAREA', 'LABEL'];
+
+    function navigate(url) {
+      var mc = document.getElementById('main-content');
+      if (mc && window.htmx) {
+        htmx.ajax('GET', url, { target: '#main-content', swap: 'innerHTML', pushUrl: url });
+      } else {
+        window.location.href = url;
+      }
+    }
+
+    document.addEventListener('touchend', function (e) {
+      // Ignore taps that land on interactive elements
+      var el = e.target;
+      while (el && el !== document.body) {
+        if (INTERACTIVE.indexOf(el.tagName) !== -1) return;
+        el = el.parentElement;
+      }
+
+      var touch    = e.changedTouches[0];
+      var side     = touch.clientX < window.innerWidth / 2 ? 'left' : 'right';
+      var now      = Date.now();
+      var gap      = now - lastTap;
+
+      if (gap < THRESHOLD && side === lastSide) {
+        // Double-tap detected
+        lastTap  = 0;
+        lastSide = '';
+        if (side === 'right' && NEXT_N) {
+          navigate('/books/' + SLUG + '/chapters/' + NEXT_N);
+        } else if (side === 'left' && PREV_N) {
+          navigate('/books/' + SLUG + '/chapters/' + PREV_N);
+        }
+      } else {
+        lastTap  = now;
+        lastSide = side;
+      }
+    }, { passive: true });
+  }());
 }());
 </script>`