Fix map fullscreen state sync, closeModal flag, remove duplicate code

Map fullscreen:
- Removed mapIsFullscreen variable, use CSS class as single source of truth
- popHandler and toggleMapFs now both check mapEl.classList
- After back-gesture collapse, expand button works on first click (not second)

closeModal:
- Sets _modalPopHandled flag before history.back()
- Prevents global popstate handler from double-processing the event
- Fixes: back from modal no longer closes the app

Removed duplicate fsChart block (caused SyntaxError)

frontend/js/main.js

@@ -814,8 +814,10 @@ function showModal(html, onSubmit) {
     const fsMap = overlay.querySelector(".sd-map-full");
     if (fsMap) {
       fsMap.classList.remove("sd-map-full");
+      // Update expand button icon
       const expandBtn = fsMap.querySelector(".sd-map-expand-btn");
       if (expandBtn) expandBtn.innerHTML = "⛶";
+      // Reset map controls and size
       try {
         const mapObj = fsMap._leafletMap;
         if (mapObj) {
@@ -828,6 +830,7 @@ function showModal(html, onSubmit) {
           }, 100);
         }
       } catch {}
+      // Stay in modal — re-push entry that back consumed
       history.pushState({ modal: true }, "");
       return;
     }
@@ -848,6 +851,7 @@ function showModal(html, onSubmit) {
   window.addEventListener("popstate", popHandler);
 
   function closeModal() {
+    _modalPopHandled = true; // prevent global handler from also processing the back
     window.removeEventListener("popstate", popHandler);
     overlay.remove();
     history.back(); // pops {modal: true}, returns to previous screen
@@ -2600,27 +2604,20 @@ async function showSessionDetail(sessionId) {
 
     // Fullscreen toggle button on map
     // Fullscreen map toggle
-    let mapIsFullscreen = false;
-
-    function collapseMap() {
-      if (!mapIsFullscreen) return;
-      mapIsFullscreen = false;
-      mapEl.classList.remove("sd-map-full");
-      fsDiv.innerHTML = "⛶";
-      setTimeout(() => { map.invalidateSize(); map.fitBounds(L.latLngBounds(allCoords).pad(0.05)); }, 100);
-    }
-
     function toggleMapFs() {
-      if (mapIsFullscreen) {
-        collapseMap();
+      if (mapEl.classList.contains("sd-map-full")) {
+        // Collapse
+        mapEl.classList.remove("sd-map-full");
+        fsDiv.innerHTML = "⛶";
         map.dragging.disable();
         map.scrollWheelZoom.disable();
+        setTimeout(() => { map.invalidateSize(); map.fitBounds(L.latLngBounds(allCoords).pad(0.05)); }, 100);
       } else {
-        mapIsFullscreen = true;
+        // Expand
         mapEl.classList.add("sd-map-full");
         fsDiv.innerHTML = "✕";
         map.dragging.enable();
-        map.scrollWheelZoom.enable(); // zoom without ctrl in fullscreen
+        map.scrollWheelZoom.enable();
         setTimeout(() => { map.invalidateSize(); map.fitBounds(L.latLngBounds(allCoords).pad(0.05)); }, 100);
       }
     }