Rewrite modal navigation: fix 5 back-gesture bugs

Root causes fixed:
1. closeModal had replaceState AFTER async history.back() — replaceState
   overwrote the current entry before back() fired, causing skip-to-today
2. Chart fullscreen in modals used pushState — each open/close cycle
   added extra history entries, exhausting the stack
3. Map collapse in popHandler didn't call invalidateSize/fitBounds

Changes:
- closeModal() simplified: just remove overlay + history.back()
  No more replaceState race condition
- Chart/map fullscreen inside modals: NO pushState, CSS-only
  popHandler detects .chart-full/.sd-map-full and collapses, re-pushes modal
- Map collapse: uses saved _leafletMap ref and _bounds for proper resize
- Init: renderScreen with fromPopstate=true to avoid navStack duplication
- Modal popHandler searches within overlay only (not global document)

Fixed user-reported bugs:
1. Map collapse keeps zoom → now calls invalidateSize + fitBounds
2. Chart open → back → back = app exit → now back closes chart, back closes modal
3. "<-" goes to Today instead of track list → fixed closeModal race
4. X on chart → back = correct → was already working
5. Chart open → back back = app exit → same fix as #2

frontend/js/main.js

@@ -94,8 +94,8 @@ async function init() {
   // Add padding entries so first back doesn't exit
   for (let i = 0; i < 3; i++) history.pushState({ screen: navStack[navStack.length - 1] }, "");
 
-  // Restore last screen
-  renderScreen(navStack[navStack.length - 1]);
+  // Restore last screen (fromPopstate=true to avoid duplicate navStack push)
+  renderScreen(navStack[navStack.length - 1], true);
 }
 
 // ---------------------------------------------------------------------------
@@ -642,18 +642,29 @@ function showModal(html, onSubmit) {
 
   history.pushState({ modal: true }, "");
   const popHandler = () => {
-    // Fullscreen map inside modal? Collapse it first.
+    // Fullscreen map inside modal? Collapse it, stay in modal.
     const fsMap = overlay.querySelector(".sd-map-full");
     if (fsMap) {
       fsMap.classList.remove("sd-map-full");
       const expandBtn = fsMap.querySelector(".sd-map-expand-btn");
       if (expandBtn) expandBtn.innerHTML = "&#x26F6;";
-      try { map.dragging.disable(); map.scrollWheelZoom.disable(); } catch {}
+      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);
+        }
+      } catch {}
       history.pushState({ modal: true }, "");
       return;
     }
-    // Fullscreen chart inside modal? Collapse it first.
-    const fsChart = document.querySelector(".chart-full");
+    // 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");
@@ -663,23 +674,21 @@ function showModal(html, onSubmit) {
     }
     // Nothing fullscreen — close modal
     window.removeEventListener("popstate", popHandler);
-    closeModal(false);
+    overlay.remove();
   };
   window.addEventListener("popstate", popHandler);
 
-  function closeModal(goBack) {
-    overlay.remove();
+  function closeModal() {
     window.removeEventListener("popstate", popHandler);
-    if (goBack) history.back();
-    // Ensure history isn't empty after modal close (prevents app exit on next back)
-    history.replaceState({ screen: state.screen }, "");
+    overlay.remove();
+    history.back(); // pops {modal: true}, returns to previous screen
   }
 
-  overlay.querySelector("#modal-back").onclick = () => closeModal(true);
-  overlay.querySelector("#modal-cancel").onclick = () => closeModal(true);
+  overlay.querySelector("#modal-back").onclick = () => closeModal();
+  overlay.querySelector("#modal-cancel").onclick = () => closeModal();
   overlay.querySelector("#modal-ok").onclick = async () => {
     const ok = await onSubmit(overlay);
-    if (ok !== false) { closeModal(true); renderScreen(state.screen); }
+    if (ok !== false) { closeModal(); renderScreen(state.screen); }
   };
   setTimeout(() => overlay.querySelector("input")?.focus(), 50);
 }
@@ -2182,6 +2191,7 @@ async function showSessionDetail(sessionId) {
 
     L.Icon.Default.imagePath = "lib/";
     const map = L.map(mapEl, { attributionControl: false, zoomControl: false, scrollWheelZoom: false, dragging: false, tap: false, touchZoom: true });
+    mapEl._leafletMap = map; // save for popHandler map collapse
     // Ctrl+wheel zoom on desktop
     mapEl.addEventListener("wheel", (e) => {
       if (e.ctrlKey) { e.preventDefault(); map.setZoom(map.getZoom() + (e.deltaY < 0 ? 1 : -1)); }
@@ -2225,6 +2235,7 @@ async function showSessionDetail(sessionId) {
     }
 
     const allCoords = sortedCoords;
+    mapEl._bounds = L.latLngBounds(sortedCoords).pad(0.1); // save for collapse fitBounds
 
     // Fullscreen toggle button on map
     // Fullscreen map toggle
@@ -2417,12 +2428,11 @@ 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.
     document.querySelectorAll(".chart-wrap").forEach(wrap => {
       wrap.onclick = () => {
         if (wrap.classList.contains("chart-full")) return;
         wrap.classList.add("chart-full");
-        // Push history so back gesture collapses chart, not modal
-        history.pushState({ chartFull: true }, "");
         const closeBtn = document.createElement("div");
         closeBtn.className = "chart-close-btn";
         closeBtn.innerHTML = "&#x2715;";