Drag-Griff statt ganzem Element + Uebungs-Reihenfolge beim Start uebernehmen

- Sortieren nur ueber einen kleinen Griff (Punkt-Icon) links im
  Uebungskopf; das ganze Element ist nicht mehr draggable (kein
  versehentliches Ziehen beim Tippen/Auswaehlen)
- Beim Session-Start wird die Uebungs-Reihenfolge der letzten
  abgeschlossenen Session desselben Tags uebernommen (server-seitig in
  create_session, in session.fields.exerciseOrder)
- Tests: 170 Checks gruen (40+18+52+32+28)

app.js

@@ -782,17 +782,16 @@ function reorderExercise(a, model, fromKey, toKey) {
   scheduleAutosave(model);
 }
 
-function attachDragReorder(a, model, ex, block) {
-  block.draggable = true;
-  block.title = t("reorder_hint");
-  block.addEventListener("dragstart", ev => {
+function attachDragReorder(a, model, ex, block, grip) {
+  grip.draggable = true;
+  grip.addEventListener("dragstart", ev => {
     app.dragKey = ex.key;
     try {
       ev.dataTransfer = ev.dataTransfer || {};
       ev.dataTransfer.setData("text/plain", ex.key);
     } catch (_) {}
   });
-  block.addEventListener("dragend", () => {
+  grip.addEventListener("dragend", () => {
     app.dragKey = null;
     for (const b of $$(".ex-block.drag-over")) b.classList.remove("drag-over");
   });
@@ -850,6 +849,14 @@ function buildExBlock(a, model, ex, removable) {
   model.exByKey[ex.key] = ex;
 
   const head = el("div", "ex-head");
+  const grip = el("span", "ex-grip", "");
+  grip.innerHTML = '<svg viewBox="0 0 16 16" width="14" height="14" fill="currentColor" aria-hidden="true">'
+    + '<circle cx="6" cy="4" r="1.4"/><circle cx="10" cy="4" r="1.4"/>'
+    + '<circle cx="6" cy="8" r="1.4"/><circle cx="10" cy="8" r="1.4"/>'
+    + '<circle cx="6" cy="12" r="1.4"/><circle cx="10" cy="12" r="1.4"/></svg>';
+  grip.title = t("reorder_hint");
+  grip.setAttribute("aria-label", t("reorder_hint"));
+  head.appendChild(grip);
   const infoBtn = el("button", "icon-btn", "?");
   infoBtn.type = "button";
   infoBtn.setAttribute("aria-label", t("info_aria") + ex.name);
@@ -878,7 +885,7 @@ function buildExBlock(a, model, ex, removable) {
 
   block.appendChild(el("div", "set-rows"));
   renderExRows(a, model, ex);
-  attachDragReorder(a, model, ex, block);
+  attachDragReorder(a, model, ex, block, grip);
   return block;
 }