Room page: colour swatches send their colour, not white

oklchToRgb read the colour back from the browser's CSS engine and expected
rgb(...). Current Chromium (the kiosk's 153) resolves an oklch colour to
oklch(...), so the match failed and every swatch fell back to [255, 255, 255]:
tapping a colour set the light to white. It now does the OKLab maths itself.

Also log a Home Assistant call that fails in the proxy: httpx only logs a
request once it has a response, so a call that timed out left no trace.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

python-backend/musicmouse/services/web/api.py

@@ -376,6 +376,10 @@ def build_router(
                 method, f"{ha.url}{path}", headers={"Authorization": f"Bearer {ha.token}"}, **kwargs
             )
         except httpx2.HTTPError as exc:
+            # httpx logs a request only once it has a response, so without this a call
+            # that times out - a Homematic command the device never acknowledges can
+            # keep Home Assistant from answering - leaves no trace at all.
+            _log.warning("Home Assistant %s %s failed: %r", method, path, exc)
             raise HTTPException(
                 status_code=502, detail=f"Home Assistant unreachable: {exc}"
             ) from exc