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/service.py

@@ -67,7 +67,12 @@ class SpiritService:
                 settings.ollama_fast_model,
                 prompts.fragment_prompt(source, anomaly, language),
                 system=prompts.fragment_system(
-                    "a spirit box" if source == "radio" else "an EVP recorder", language
+                    {
+                        "radio": "a spirit box",
+                        "evp": "an EVP recorder",
+                        "emf": "an EMF field meter",
+                    }.get(source, "the veil"),
+                    language,
                 ),
                 options={"num_predict": 16, "temperature": 0.95},
             )