Avoid map zoom-out when navigating between routes

Route links (route_master list, other directions) now intercept clicks
and call loadRoute() + history.pushState() instead of doing a full page
reload, so the map stays at its current position and zooms smoothly into
the new route. Ctrl/cmd/middle-click still opens in a new tab via href.

Introduce currentRelationId (mutable) to track the loaded relation so
loadSegment() uses the correct ID after in-page navigation.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/static/app.js

@@ -22,6 +22,7 @@ let routeMasterLayers = [];  // coloured polylines when viewing a route_master
 
 // ── State ─────────────────────────────────────────────────────────────────
 
+let currentRelationId = RELATION_ID;  // tracks the currently loaded relation
 let routeData = null;        // response from /api/route/
 let segmentGeoJson = null;   // last response from /api/segment/ (always includes stops)
 let activeSlot = 'from';     // 'from' | 'to' | null
@@ -301,6 +302,21 @@ document.getElementById('include-stops').addEventListener('change', () => {
 document.getElementById('slot-from').addEventListener('click', () => setActiveSlot('from'));
 document.getElementById('slot-to').addEventListener('click', () => setActiveSlot('to'));
 
+// ── Navigation ─────────────────────────────────────────────────────────────
+
+/**
+ * Load a relation without a full page reload, updating the browser URL.
+ * Allows middle-click / ctrl-click to still open in a new tab via the href.
+ * @param {number} id
+ * @param {MouseEvent} e
+ */
+function navigateTo(id, e) {
+  if (e.metaKey || e.ctrlKey || e.shiftKey || e.button !== 0) return;
+  e.preventDefault();
+  history.pushState(null, '', `/${id}`);
+  loadRoute(id);
+}
+
 // ── API calls ──────────────────────────────────────────────────────────────
 
 /**
@@ -320,6 +336,7 @@ async function loadRoute(relationId) {
       return;
     }
     routeData = data;
+    currentRelationId = relationId;
     // Clean up any previous route_master view
     for (const l of routeMasterLayers) l.remove();
     routeMasterLayers = [];
@@ -358,6 +375,7 @@ async function loadRoute(relationId) {
         a.href = `/${dir.id}`;
         a.className = 'stop-item d-block text-decoration-none';
         a.textContent = dir.name;
+        a.addEventListener('click', (e) => navigateTo(dir.id, e));
         dirList.appendChild(a);
       }
     } else {
@@ -373,7 +391,7 @@ async function loadRoute(relationId) {
  */
 async function loadSegment() {
   if (!selectedFrom || !selectedTo || !routeData) return;
-  const rid = RELATION_ID;
+  const rid = currentRelationId;
   const params = new URLSearchParams({ from: selectedFrom, to: selectedTo, stops: '1' });
   try {
     const resp = await fetch(`/api/segment/${rid}?${params}`);
@@ -444,6 +462,7 @@ async function loadRouteMaster(relationId) {
       a.href = `/${route.id}`;
       a.className = 'text-decoration-none text-reset flex-grow-1';
       a.textContent = route.name;
+      a.addEventListener('click', (e) => navigateTo(route.id, e));
 
       div.appendChild(dot);
       div.appendChild(a);