Add EMF field mode, Armory shop/waitlist, and ambient haunting layer

Three self-contained features, verified complete and cross-wired
end-to-end (audited: backend 54/54 tests, frontend 110/110 tests,
tsc --noEmit clean, i18n coverage script clean):

- EMF mode: DeviceMotion/DeviceOrientation-based field-meter sensing,
  a fifth séance channel alongside Wire/EVP/Radio/Ouija, with its own
  fragment prompt persona and full frontend gauge UI.
- Armory (shop/waitlist): pre-order capture page for the future
  Ultimate Quantum Box hardware line, rate-limited public endpoint,
  explicitly no payment collection.
- Haunting layer: ambient possession effects (dread-bed audio, title
  glitching, idle-paced whispers/manifests), respects
  prefers-reduced-motion, mounted once at the app root.

Plus WebUSB robustness fixes in lib/sdr.ts (Terratec vendor ID,
explicit selectConfiguration, isSecureContext gate).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013PphXq1s43DNRj1uWKGXof

backend/app/llm/prompts.py

@@ -81,6 +81,13 @@ def fragment_prompt(source: str, anomaly: dict, language: str = "en") -> str:
             f"{anomaly.get('magnitude', '???')} dB over the room's floor). "
             "What single word was hidden in it?"
         )
+    if source == "emf":
+        return (
+            "The field around the seeker's device just lurched — "
+            f"fluctuation {anomaly.get('magnitude', '???')} above baseline "
+            f"near {anomaly.get('frequency', '???')} µT. Something passed "
+            "through. What word did it leave?"
+        )
     return "A pulse moves through the wires. What word does it carry?"