Smart Speed Phase 1: add Web Audio API pipeline skeleton and enableSmartSpeed setting

- Wrap HTML5 <audio> in AudioContext + MediaElementAudioSourceNode in
  LocalAudioPlayer.js (non-breaking; falls back gracefully)
- Add enableSmartSpeed: false to user settings store (client-side)
- Add SMART_SPEED_PHASE1_DESIGN.md design document

client/store/user.js

@@ -18,7 +18,8 @@ export const state = () => ({
     authorSortBy: 'name',
     authorSortDesc: false,
     jumpForwardAmount: 10,
-    jumpBackwardAmount: 10
+    jumpBackwardAmount: 10,
+    enableSmartSpeed: false
   }
 })