Add Start button and Home/End keys to editor for navigation

After auto-scroll during preview, users couldn't easily navigate back
to the beginning to reposition the play-from marker. Added:
- "⏮ Start" button that resets scroll and play-from marker to beginning
- Home key shortcut to do the same
- End key shortcut to jump to end of piece

https://claude.ai/code/session_01TLvDyKzUQekd9hzFuEJ5YX

index.html

@@ -197,6 +197,7 @@ canvas{display:block;}
     <label style="color:#aaa;font-size:12px;">BPM: <input type="number" id="ed-bpm" value="120" min="30" max="300" style="width:55px;background:#1a1a2e;color:#fff;border:1px solid #444;padding:2px 4px;border-radius:3px;" onchange="edUpdateBpm()"></label>
     <label style="color:#aaa;font-size:12px;">Time: <select id="ed-timesig" style="background:#1a1a2e;color:#fff;border:1px solid #444;padding:2px;border-radius:3px;font-size:12px;" onchange="edUpdateTimeSig()"><option>4/4</option><option>3/4</option><option>2/4</option><option>6/8</option><option>9/8</option><option>5/4</option><option>7/8</option><option>12/8</option></select></label>
     <label style="color:#aaa;font-size:12px;">Zoom: <input type="range" id="ed-zoom" min="20" max="200" value="80" style="width:80px;" oninput="edDraw()"></label>
+    <button class="smbtn" onclick="edGoToStart()" title="Scroll to start and reset play-from marker (Home key)">⏮ Start</button>
     <button class="smbtn" onclick="edPreview()" id="ed-pv-btn">▶ Preview</button>
     <button class="smbtn" onclick="edSave()" style="background:#38a169;">💾 Save</button>
     <button class="smbtn" onclick="edExport()">📋 Copy JSON</button>
@@ -5614,6 +5615,17 @@ function edKeyDown(e){
     edSelRange=null;edSelected=-1;edPlayFromMarker=-1;edDraw();e.preventDefault();return;
   }
 
+  // Home key — scroll to start and reset play-from marker
+  if(e.key==="Home"){edGoToStart();e.preventDefault();return;}
+
+  // End key — scroll to end of piece
+  if(e.key==="End"){
+    var cv=getEdCanvas();
+    var endT=edIsClassical?(edPiece&&edPiece.duration||90):4;
+    edScrollX=Math.max(0,endT*edPxPerUnit()-cv.width+50);
+    edDraw();e.preventDefault();return;
+  }
+
   if((e.key==="Delete"||e.key==="Backspace")&&edSelected>=0){
     edMelody.splice(edSelected,1);
     edSelected=-1;
@@ -5753,6 +5765,12 @@ function edDurInfo(durSec){
   return {sym:"\u266C\u266C",name:"32nd",val:0.125};
 }
 
+function edGoToStart(){
+  edScrollX=0;
+  edPlayFromMarker=-1;
+  edDraw();
+}
+
 function edStopPlayhead(){
   edPlayhead=-1;
   if(edPlayheadRAF)cancelAnimationFrame(edPlayheadRAF);