chore(ts): drop two more redundant casts

- RouteMapThumbnail.client.tsx: `{ type: 'Feature', ... } as unknown
  as GeoJsonObject` → `as GeoJsonObject`. The literal already
  satisfies the type; the unknown bridge was unnecessary.

- use-yjs.ts:102: keep the coercion (y-websocket's `.on()` signature
  doesn't include the legacy `"synced"` event even though the
  runtime still fires it), but document why with a comment so future
  readers don't try to drop it.

apps/journal/app/components/RouteMapThumbnail.client.tsx

@@ -142,7 +142,7 @@ function DayColoredRoute({ data, dayBreaks, highlightedDay }: { data: GeoJsonObj
           type: "Feature",
           geometry: { type: "LineString", coordinates: seg.coords },
           properties: {},
-        } as unknown as GeoJsonObject;
+        } as GeoJsonObject;
         return (
           <GeoJSON
             key={`${i}-${highlightedDay}`}