feat: home button to center view + fix sequencer step count growth

- Add ⌂ button to zoom bar (sandbox + puzzle) that centers camera on
  all modules
- Fix sequencer _steps array not growing when step count param increases
  (e.g. 8→32 now properly adds new empty steps)
- Make piano roll width dynamic based on bar count (BEAT_PX constant
  density instead of fixed ROLL_W)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/App.jsx

@@ -191,6 +191,26 @@ export default function App({ onSwitchToGame }) {
     emit();
   }, []);
 
+  // Center view on all modules
+  const handleCenterView = useCallback(() => {
+    if (state.modules.length === 0) { state.camX = 0; state.camY = 0; emit(); return; }
+    const container = containerRef.current;
+    const cw = container?.clientWidth || 800;
+    const ch = container?.clientHeight || 600;
+    let minX = Infinity, minY = Infinity, maxX = -Infinity, maxY = -Infinity;
+    for (const m of state.modules) {
+      minX = Math.min(minX, m.x);
+      minY = Math.min(minY, m.y);
+      maxX = Math.max(maxX, m.x + 200);
+      maxY = Math.max(maxY, m.y + 150);
+    }
+    const cx = (minX + maxX) / 2 * state.zoom;
+    const cy = (minY + maxY) / 2 * state.zoom;
+    state.camX = cw / 2 - cx;
+    state.camY = ch / 2 - cy;
+    emit();
+  }, []);
+
   const handleToggleAudio = async () => {
     if (state.isRunning) {
       stopAudio();
@@ -305,6 +325,7 @@ export default function App({ onSwitchToGame }) {
             {(state.zoom * 100).toFixed(0)}%
           </button>
           <button className="zoom-btn" onClick={handleZoomOut} title="Zoom out">−</button>
+          <button className="zoom-btn" onClick={handleCenterView} title="Centrar vista">⌂</button>
         </div>
 
         {/* Module palette */}