Jump to the middle; bold jump arrows; Chicago time; demo hours 6 AM-9 PM

- Jump sheet: Top / Middle / Bottom with the Card fields arrows; the Jump
  button uses the same bold arrows.
- Container timezone defaults to America/Chicago (TZ in Dockerfile/compose).
- Demo worker follows its own business hours (Demo.BusinessHours, default
  06:00-21:00 every day) with the midnight pitstop; the simulated cycle in
  the browser shows the real worker's next run.

Dockerfile

@@ -12,7 +12,7 @@ ENV USMT_MODE=Demo \
     POWERSHELL_TELEMETRY_OPTOUT=1 \
     POWERSHELL_UPDATECHECK=Off \
     DOTNET_CLI_TELEMETRY_OPTOUT=1 \
-    TZ=UTC
+    TZ=America/Chicago
 
 WORKDIR /app
 COPY . /app