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/engine/audioEngine.js

@@ -170,7 +170,8 @@ function createNode(mod) {
         },
       };
     }
-    case 'keyboard': {
+    case 'keyboard':
+    case 'drumpad': {
       const freqSig = new Tone.Signal(440);
       const gateSig = new Tone.Signal(0);
       return {
@@ -251,7 +252,7 @@ export function connectWire(conn) {
   // set the oscillator frequency directly when notes are played.
   const fromMod = state.modules.find(m => m.id === conn.from.moduleId);
   const toMod = state.modules.find(m => m.id === conn.to.moduleId);
-  if (fromMod && ['keyboard', 'sequencer', 'pianoroll'].includes(fromMod.type) &&
+  if (fromMod && ['keyboard', 'drumpad', 'sequencer', 'pianoroll'].includes(fromMod.type) &&
       conn.from.port === 'freq' && toMod?.type === 'oscillator' && conn.to.port === 'freq') {
     return; // handled imperatively in triggerKeyboard / setSequencerSignals
   }
@@ -354,6 +355,7 @@ export function updateParam(moduleId, paramName, value) {
       if (paramName === 'volume') entry.node.gain.value = Tone.dbToGain(value);
       break;
     case 'keyboard':
+    case 'drumpad':
     case 'sequencer':
     case 'pianoroll':
       // All params stored in state, managed by widgets