fix(app): escape chart data in the standalone chart export

[asasemahmed]

chartToStandaloneHtml inserts labels, values and colors into an inline <script> with plain JSON.stringify, which leaves </script> as is. A chart label containing </script> therefore closes the script tag, and the exported file either breaks or runs whatever follows. The labels come from model output. assembleStandaloneHtmlFromActivity already escapes </script, so the two exports behave differently.

The values now go through a small toScriptJson helper that escapes < as <. It is the same value in JS, but it cannot close the tag.

Tests: a label of </script><script>...</script> leaves only the two expected </script> tags in the output.