feat: standalone preview.html — visual demo without any install

[Copilot]

No standalone file existed for previewing the UI without running the backend stack.

Adds preview.html at the repo root — a single self-contained file with all CSS/JS inlined and a built-in demo data layer. Open it directly in any browser; no server, Node.js, or npm required.

Demo mode

• 5 simulated signals (3 Wi-Fi + 2 BLE) rendered immediately on load
• DoA and RSSI drift continuously to keep animations live
• Active/idle state toggles periodically
• Auto-locates the selected target after 8 s, triggering the full located-circle + banner animation
• Switching targets resets the countdown

Implementation notes

• Renderer class inlined verbatim from frontend/js/renderer.js
• WsClient replaced with a tickDemo() function called every 250 ms via setTimeout
• Base DoA/RSSI cached on signal objects at hydration time to avoid repeated find() in the tick loop
• currentTargetId kept module-scoped (not on window), shared via a setCurrentTargetId() setter