Implement Sound side-panel section (master audio on/off + volume)

[vecnode]

Summary

Implements the previously proposal-only "Sound" section (docs/proposals/sound-section.md) - a new panel between Sketch and Libraries with a master Off/On toggle and a 0-1 volume slider over sketch audio, muted by default.

• buildAudioController(enabled, volume) in script.js is injected into the sketch iframe (after captureController, before any imported library/the sketch code). It wraps AudioContext/webkitAudioContext once: each context gets a GainNode spliced in via Reflect.construct(RealAC, arguments), with destination shadowed as an own property on that instance - so anything the sketch connects to ctx.destination (p5.sound, raw Web Audio, an imported library) lands on the gain node instead of real output.
• applyAudioState() posts { type: 'audio-set', enabled, volume } to the running sketch so toggling/adjusting the slider takes effect immediately, without re-running.
• No new capability crosses the sandbox boundary - the message is just a boolean and a float, iframe stays sandbox="allow-scripts" with no allow-same-origin, same as everywhere else in the app.
• Proposal doc updated with a "Status: implemented" note listing the (small) differences from the original design.

Test plan

• node --check public/script.js passes; extracted buildAudioController's template and ran it through vm.Script to confirm the injected string itself is valid JS.
• Built and ran locally. In the live app:
  • Sound section renders between Sketch and Libraries; defaults to Off, slider disabled, matches the "muted by default" design.
  • Ran a sketch that creates a real AudioContext + oscillator: ctx.destination.constructor.name === 'GainNode' and its initial gain.value matches the baked-in enabled/volume state.
  • Toggled Sound On/Off and moved the slider on an already-running sketch (no re-run) and confirmed ctx.destination.gain.value updates live each time (1 -> 0 -> 0.3) via the postMessage bridge.
  • Confirmed light/dark theme and the Ace dark-theme fix from the previous two rounds still work correctly alongside this change; no console errors.

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