feat(sequencer): Add velocity control and pattern management

Add comprehensive beat sequencer enhancements:

Velocity System:
- 4-level velocity control (soft/normal/accent) with visual feedback
- Left-click toggle, right-click cycle velocity
- Dynamic opacity and glow effects for accents

Pattern Management:
- Clear/reset functions for quick editing
- 4 musical presets (Rock, Funk, Techno, Breakbeat)
- LocalStorage save/load/delete with persistent storage

UI/UX:
- Color-coded preset buttons and save/load panel
- Velocity instruction guide
- Fixed input visibility and playback indicators

Technical: 10 functions, 12 UI components, ~240 LOC

Author: burinc

src/scittle/audio/audio_playground.clj

@@ -1,3 +1,4 @@
+
 ^:kindly/hide-code
 ^{:kindly/options {:html/deps [:scittle :reagent]}
   :clay {:title "Web Audio API Playground with ClojureScript & Scittle"