feat: legibility wave — veil conditions, first-run hints, kinder errors

Workstream B of the usability wave (#2 hints, #3 conditions, #4 errors):

- GET /api/conditions (new backend/app/routes/conditions.py): composes
  celestial veil_thinness with the cached NOAA Kp reading; lenient lon
  parsing (junk degrades to moon-only, never 422); geomagnetic may be
  null on a cold cache. Route tests stub the cache — no live NOAA calls.
- VeilConditions strip in the séance side column: moon glyph + phase,
  % lit, veil-thinness phrase, Kp line only when data exists. Polls
  every 10 min; renders nothing while loading; no error state.
- ModeHint: per-mode in-fiction one-liner after ~15s idle, suppressed
  once the mode's sensor runs this session, dismissal persisted in
  localStorage (qm_hint_<mode>). One mount line per panel.
- Error copy upgraded to detect-and-redirect: mic denied points at site
  settings and the ouija board/wire; WebUSB-unsupported suggests EVP.
- i18n en/es parity for every new string; coverage-check template
  domains extended for the new template-key call sites.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/app/routes/conditions.py

@@ -0,0 +1,46 @@
+"""Veil conditions readout — the sky and the field, in one call.
+
+Composes the two real-world signals the app already computes:
+
+  sky          - app.celestial.veil_thinness(): moon phase/illumination and
+                 (when a longitude is offered) proximity to true solar
+                 midnight. Pure math, always present.
+  geomagnetic  - app.geomagnetic.geomagnetic_cache.reading(): NOAA's
+                 planetary Kp from cache. May be None on a cold cache or
+                 when NOAA is unreachable — that is a valid answer, not an
+                 error, and the frontend simply shows fewer lines.
+
+`lon` is accepted as a raw string and parsed by hand so a malformed or
+out-of-range value degrades to the moon-only reading instead of a 422 —
+the conditions strip must never fail because a browser sent junk.
+"""
+
+import math
+
+from fastapi import APIRouter
+
+from app.celestial import veil_thinness
+from app.geomagnetic import geomagnetic_cache
+
+router = APIRouter(prefix="/api", tags=["conditions"])
+
+
+def _parse_longitude(raw: str | None) -> float | None:
+    """A usable longitude in [-180, 180], or None for anything else."""
+    if raw is None:
+        return None
+    try:
+        lon = float(raw)
+    except (TypeError, ValueError):
+        return None
+    if not math.isfinite(lon) or not -180.0 <= lon <= 180.0:
+        return None
+    return lon
+
+
+@router.get("/conditions")
+async def get_conditions(lon: str | None = None):
+    return {
+        "sky": veil_thinness(_parse_longitude(lon)),
+        "geomagnetic": await geomagnetic_cache.reading(),
+    }