feat: Signal UX redesign

[msitarzewski]

Summary

• Complete visual redesign of the web client with the "Signal" design system — an atmospheric broadcast studio experience inspired by underground radio
• New color palette (void/signal/data), typography (Space Grotesk / Inter / JetBrains Mono), and atmospheric effects (scan lines, vignette, noise texture)
• ON AIR animations, segmented LED meters, waveform oscilloscope, speaking detection, collapsible deck panels, channel strip participant cards
• Local mic now routed into program bus so Signal Output shows the complete broadcast mix
• All 3 E2E tests passing (WebRTC, Recording, Return Feed)

Test plan

• Load http://localhost:6736 and confirm Signal design renders (dark atmosphere, amber wordmark, scan lines)
• Start Broadcast → verify ON AIR state (red signal line, red wordmark, red card accents)
• Speak into mic → verify waveform oscilloscope responds, LED meter lights up, participant card glows amber
• Second peer joins → verify card entrance animation (tune-in), speaking detection on both cards
• Peer leaves → verify card exit animation (tune-out)
• Record → verify REC blink, deck panel auto-expands, timer runs
• End Broadcast → verify all animations reset, empty state shows "The frequency is clear."
• Responsive: resize to 768px and 480px breakpoints
• prefers-reduced-motion: enable in DevTools → verify no animations
• node tests/test-webrtc.mjs && node tests/test-recording.mjs && node tests/test-return-feed.mjs

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