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/tests/test_conditions.py

@@ -0,0 +1,73 @@
+"""GET /api/conditions — sky always present, geomagnetic nullable, lon lenient.
+
+The geomagnetic cache is stubbed in every test so nothing here ever
+touches NOAA; a cold real cache would otherwise fire a live HTTP request.
+"""
+
+import pytest
+
+import app.routes.conditions as conditions_module
+
+
+class _StubCache:
+    def __init__(self, reading):
+        self._reading = reading
+
+    async def reading(self):
+        return self._reading
+
+
+@pytest.fixture
+def geo_null(monkeypatch):
+    monkeypatch.setattr(conditions_module, "geomagnetic_cache", _StubCache(None))
+
+
+@pytest.fixture
+def geo_storm(monkeypatch):
+    monkeypatch.setattr(
+        conditions_module,
+        "geomagnetic_cache",
+        _StubCache({"kp": 6.33, "label": "moderate geomagnetic storm",
+                    "disturbance": 6.33 / 9.0, "storm": True}),
+    )
+
+
+@pytest.mark.asyncio
+async def test_sky_always_present_geomagnetic_null_tolerated(client, geo_null):
+    resp = await client.get("/api/conditions")
+    assert resp.status_code == 200
+    body = resp.json()
+    assert body["geomagnetic"] is None
+    sky = body["sky"]
+    assert sky["moon_name"]
+    assert 0.0 <= sky["moon_illumination"] <= 1.0
+    assert 0.0 <= sky["thinness"] <= 1.0
+    # No longitude offered — witching proximity is unknowable.
+    assert sky["witching_proximity"] is None
+
+
+@pytest.mark.asyncio
+async def test_valid_lon_yields_witching_proximity(client, geo_null):
+    resp = await client.get("/api/conditions", params={"lon": "-3.7"})
+    assert resp.status_code == 200
+    witching = resp.json()["sky"]["witching_proximity"]
+    assert witching is not None
+    assert 0.0 <= witching <= 1.0
+
+
+@pytest.mark.asyncio
+@pytest.mark.parametrize("bad", ["ghost", "", "nan", "inf", "999", "-181"])
+async def test_invalid_lon_ignored_not_422(client, geo_null, bad):
+    resp = await client.get("/api/conditions", params={"lon": bad})
+    assert resp.status_code == 200
+    assert resp.json()["sky"]["witching_proximity"] is None
+
+
+@pytest.mark.asyncio
+async def test_geomagnetic_reading_passed_through(client, geo_storm):
+    resp = await client.get("/api/conditions")
+    assert resp.status_code == 200
+    geo = resp.json()["geomagnetic"]
+    assert geo["kp"] == pytest.approx(6.33)
+    assert geo["storm"] is True
+    assert geo["label"] == "moderate geomagnetic storm"