Rewrite fullscreen as history entries: each open=push, each back=pop

Every fullscreen (map/chart) inside a modal now pushes its own history
entry via overlay._pushFullscreen(). Back pops it, popHandler collapses.
No more re-pushing, no flag juggling between handlers.

History stack trace:
  click track → push {modal}
  expand map  → push {modalFs}
  back        → pop {modalFs}, collapse map, stay in modal
  back        → pop {modal}, close modal, return to track list
  back        → pop {screen:gps}, return to previous screen

Map toggle: expand pushes via _pushFullscreen, collapse calls history.back()
Chart toggle: same pattern — push on expand, back on X button
closeModal: pops all entries this modal owns (_historyDepth counter)

Fixes: back from fullscreen map/chart no longer closes app,
expand button works correctly after back-collapse

frontend/js/main.js

@@ -808,53 +808,60 @@ function showModal(html, onSubmit) {
     </div>`;
   document.body.appendChild(overlay);
 
+  // Simple history stack: each open = pushState, each back = popState pops it.
+  // Modal = 1 entry. Fullscreen map/chart inside modal = 1 more entry each.
+  // Back always pops the top entry. No re-pushing, no flags.
+  let _historyDepth = 1; // how many entries this modal owns
   history.pushState({ modal: true }, "");
+
   const popHandler = () => {
-    // Fullscreen map inside modal? Collapse it, stay in modal.
+    _historyDepth--;
+
+    // Check what's on top and collapse it
     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 = "&#x26F6;";
-      // Reset map controls and size
       try {
         const mapObj = fsMap._leafletMap;
         if (mapObj) {
           mapObj.dragging.disable();
           mapObj.scrollWheelZoom.disable();
-          setTimeout(() => {
-            mapObj.invalidateSize();
-            const bounds = fsMap._bounds;
-            if (bounds) mapObj.fitBounds(bounds);
-          }, 100);
+          setTimeout(() => { mapObj.invalidateSize(); if (fsMap._bounds) mapObj.fitBounds(fsMap._bounds); }, 100);
         }
       } catch {}
-      // Stay in modal — re-push entry that back consumed
-      history.pushState({ modal: true }, "");
-      return;
+      return; // stay in modal, depth already decremented
     }
-    // Fullscreen chart inside modal? Collapse it, stay in modal.
+
     const fsChart = overlay.querySelector(".chart-full");
     if (fsChart) {
       fsChart.classList.remove("chart-full");
       const btn = fsChart.querySelector(".chart-close-btn");
       if (btn) btn.remove();
-      history.pushState({ modal: true }, "");
-      return;
+      return; // stay in modal
     }
-    // Nothing fullscreen — close modal. Set flag so global handler skips this event.
+
+    // Nothing fullscreen — close modal
     _modalPopHandled = true;
     window.removeEventListener("popstate", popHandler);
     overlay.remove();
   };
   window.addEventListener("popstate", popHandler);
 
+  // Called when fullscreen opens inside this modal — push 1 entry
+  overlay._pushFullscreen = function() {
+    _historyDepth++;
+    history.pushState({ modalFs: true }, "");
+  };
+
   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
+    // Pop all entries this modal owns
+    if (_historyDepth > 0) {
+      for (let i = 0; i < _historyDepth; i++) history.back();
+    }
   }
 
   overlay.querySelector("#modal-back").onclick = () => closeModal();
@@ -2606,19 +2613,18 @@ async function showSessionDetail(sessionId) {
     // Fullscreen map toggle
     function toggleMapFs() {
       if (mapEl.classList.contains("sd-map-full")) {
-        // Collapse
-        mapEl.classList.remove("sd-map-full");
-        fsDiv.innerHTML = "&#x26F6;";
-        map.dragging.disable();
-        map.scrollWheelZoom.disable();
-        setTimeout(() => { map.invalidateSize(); map.fitBounds(L.latLngBounds(allCoords).pad(0.05)); }, 100);
+        // Collapse via history.back() — popHandler will handle the CSS
+        history.back();
       } else {
-        // Expand
+        // Expand — push to history stack
         mapEl.classList.add("sd-map-full");
         fsDiv.innerHTML = "&#x2715;";
         map.dragging.enable();
         map.scrollWheelZoom.enable();
         setTimeout(() => { map.invalidateSize(); map.fitBounds(L.latLngBounds(allCoords).pad(0.05)); }, 100);
+        // Push fullscreen entry — back will pop it
+        const modal = mapEl.closest(".modal-overlay");
+        if (modal && modal._pushFullscreen) modal._pushFullscreen();
       }
     }
 
@@ -2786,18 +2792,19 @@ async function showSessionDetail(sessionId) {
     if (exp) exp.onclick = () => GPS.exportGPX(sessionId);
 
     // Chart fullscreen toggle — tap to expand, tap X or back to collapse
-    // Chart fullscreen in modal — NO pushState. Modal popHandler handles back.
+    // Chart fullscreen in modal — pushState so back collapses it
     document.querySelectorAll(".chart-wrap").forEach(wrap => {
       wrap.onclick = () => {
         if (wrap.classList.contains("chart-full")) return;
         wrap.classList.add("chart-full");
+        const modal = wrap.closest(".modal-overlay");
+        if (modal && modal._pushFullscreen) modal._pushFullscreen();
         const closeBtn = document.createElement("div");
         closeBtn.className = "chart-close-btn";
         closeBtn.innerHTML = "&#x2715;";
         closeBtn.onclick = (e) => {
           e.stopPropagation();
-          wrap.classList.remove("chart-full");
-          closeBtn.remove();
+          history.back(); // popHandler will collapse
         };
         wrap.appendChild(closeBtn);
       };