frontend: Bereiche per Splitter (Drag) aufteilen

Verschiebbarer Gutter zwischen Editor und Diagramm:
- seitlich (vertikal/kompakt): zieht die Editor-Spaltenbreite (--col).
- gestapelt (horizontal): begrenzt die Diagrammhöhe (--diagram-h), das
  Diagramm scrollt dann intern.

Umgesetzt über ein Gutter-Grid-Feld und Pointer-Events (mit
setPointerCapture); Grenzen via min/max. Kein neues Framework.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/index.html

@@ -34,22 +34,44 @@
   header p{margin-top:4px;color:var(--muted);font-size:.9rem}
 
   /* Horizontal (Toggle aus): Diagramm oben, Editor unten — je volle Breite.
-     Vertikal (Toggle an, .side): Editor links, Diagramm rechts. */
+     Vertikal (Toggle an, .side): Editor links, Diagramm rechts.
+     Zwischen beiden liegt ein verschiebbarer Gutter (Splitter). */
   .app{
     max-width:1200px;margin:0 auto;
-    display:grid;grid-template-columns:1fr;gap:20px;
-    grid-template-areas:"diagram" "editor";
+    display:grid;gap:0;
+    grid-template-columns:1fr;
+    grid-template-areas:"diagram" "gutter" "editor";
     align-items:start;
   }
   .app.side{
-    grid-template-columns:360px 1fr;
-    grid-template-areas:"editor diagram";
+    grid-template-columns:var(--col,360px) 14px 1fr;
+    grid-template-areas:"editor gutter diagram";
   }
   .panel.editor{grid-area:editor}
   .panel.right{grid-area:diagram}
+  .gutter{grid-area:gutter}
+  /* Im gestapelten Modus die Diagrammhöhe per Gutter begrenzen (scrollt intern). */
+  .app:not(.side) .diagram{max-height:var(--diagram-h,none)}
   @media (max-width:820px){
-    .app.side{grid-template-columns:1fr;grid-template-areas:"editor" "diagram"}
+    .app.side{
+      grid-template-columns:1fr;
+      grid-template-areas:"diagram" "gutter" "editor";
     }
+  }
+
+  /* Splitter/Gutter: stapelweise horizontal, seitlich vertikal. */
+  .gutter{
+    position:relative;align-self:stretch;justify-self:stretch;
+    height:14px;cursor:row-resize;touch-action:none;
+  }
+  .app.side .gutter{height:auto;width:14px;cursor:col-resize}
+  .gutter::before{
+    content:'';position:absolute;left:50%;top:50%;transform:translate(-50%,-50%);
+    width:46px;height:4px;border-radius:999px;background:rgba(36,52,71,.16);
+    transition:background .15s;
+  }
+  .app.side .gutter::before{width:4px;height:46px}
+  .gutter:hover::before,.gutter.dragging::before{background:var(--or)}
 
   /* Editor-Innenaufbau: Textarea + aufklappbare Agenda.
      Horizontal nebeneinander, vertikal (.side) untereinander. */
@@ -145,7 +167,7 @@
 
   .diagram{
     padding:28px 16px 24px;
-    overflow-x:auto;
+    overflow:auto;
     background:transparent;
   }
   .panel.right{background:rgba(255,255,255,.6)}
@@ -362,6 +384,8 @@
     </div>
   </div>
 
+  <div class="gutter" id="gutter" role="separator" aria-label="Bereiche größenverändern" title="Ziehen zum Verschieben"></div>
+
   <div class="panel right">
     <div class="panel-head">
       <span>Diagramm</span>
@@ -551,6 +575,40 @@ document.querySelectorAll('input[name="layout"]').forEach(radio => {
   radio.addEventListener('change', () => { if(radio.checked) applyLayout(radio.value); });
 });
 
+/* ---------- Splitter: Bereiche per Drag verteilen ---------- */
+const gutter = document.getElementById('gutter');
+const diagramEl = document.querySelector('.diagram');
+let dragging = false;
+gutter.addEventListener('pointerdown', e => {
+  dragging = true;
+  gutter.classList.add('dragging');
+  gutter.setPointerCapture(e.pointerId);
+  document.body.style.userSelect = 'none';
+  e.preventDefault();
+});
+gutter.addEventListener('pointermove', e => {
+  if(!dragging) return;
+  if(app.classList.contains('side')){
+    const rect = app.getBoundingClientRect();
+    let w = e.clientX - rect.left;
+    w = Math.max(220, Math.min(rect.width - 280, w));
+    app.style.setProperty('--col', w + 'px');
+  } else {
+    let h = e.clientY - diagramEl.getBoundingClientRect().top;
+    h = Math.max(140, h);
+    app.style.setProperty('--diagram-h', h + 'px');
+  }
+});
+function endDrag(e){
+  if(!dragging) return;
+  dragging = false;
+  gutter.classList.remove('dragging');
+  document.body.style.userSelect = '';
+  try{ gutter.releasePointerCapture(e.pointerId); }catch(_){}
+}
+gutter.addEventListener('pointerup', endDrag);
+gutter.addEventListener('pointercancel', endDrag);
+
 const showc = document.getElementById('showc');
 showc.addEventListener('change', render);