feat: fullscreen keyboard + new Drum Pad module

Keyboard fullscreen:
- Double-tap keyboard widget to enter fullscreen piano mode
- 2-octave touch-friendly piano with labeled keys
- Active key highlights cyan, close button to exit

Drum Pad module (🥁):
- New module type with 4x4 colored pad grid
- Each pad triggers a unique frequency (C2-D4 range)
- Outputs freq + gate signals (same as keyboard)
- Double-tap for fullscreen pad mode with large touch targets
- Color-coded pads with hit animation

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/components/ModuleNode.jsx

@@ -5,6 +5,7 @@ import { updateParam } from '../engine/audioEngine.js';
 import Knob from './Knob.jsx';
 import ScopeDisplay from './ScopeDisplay.jsx';
 import KeyboardWidget from './KeyboardWidget.jsx';
+import DrumPadWidget from './DrumPadWidget.jsx';
 import SequencerWidget from './SequencerWidget.jsx';
 import PianoRollWidget from './PianoRollWidget.jsx';
 
@@ -248,6 +249,9 @@ export default function ModuleNode({ mod, zoom, onStartConnect, onPortPosition }
         {/* Keyboard widget */}
         {mod.type === 'keyboard' && <KeyboardWidget moduleId={mod.id} />}
 
+        {/* Drum Pad widget */}
+        {mod.type === 'drumpad' && <DrumPadWidget moduleId={mod.id} />}
+
         {/* Sequencer widget */}
         {mod.type === 'sequencer' && <SequencerWidget moduleId={mod.id} />}