feat(Q4): kiosk ambient mode with camera rotation, alert takeover, dimming

Add GET /kiosk/ambient route and standalone fullscreen template with
rotating camera snapshots (crossfade), top bar clock/date, pet status
bottom bar, SSE-driven alert takeover with HLS playback, configurable
screen dimming, and 6-hour auto-refresh.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/unit/test_kiosk_ambient.py

@@ -0,0 +1,29 @@
+import pytest
+from vigilar.config import VigilarConfig
+from vigilar.web.app import create_app
+
+
+@pytest.fixture
+def kiosk_app():
+    cfg = VigilarConfig()
+    app = create_app(cfg)
+    app.config["TESTING"] = True
+    return app
+
+
+def test_ambient_route_exists(kiosk_app):
+    with kiosk_app.test_client() as c:
+        rv = c.get("/kiosk/ambient")
+        assert rv.status_code == 200
+
+
+def test_ambient_contains_clock(kiosk_app):
+    with kiosk_app.test_client() as c:
+        rv = c.get("/kiosk/ambient")
+        assert b"clock" in rv.data
+
+
+def test_ambient_contains_hls(kiosk_app):
+    with kiosk_app.test_client() as c:
+        rv = c.get("/kiosk/ambient")
+        assert b"hls.min.js" in rv.data