docs: correct HTML chart Handlebars bindings and sandbox limitations

[igorlukanin]

The HTML chart docs page describes a template context and a JavaScript capability that don't match what the product does. Every code example on the page currently fails if copy-pasted.

Handlebars bindings. The page documents result._first and {{#each result.data}}. There's no result key in the template context — rows are a flat data array. Templates compile in strict mode, so these don't render blank, they throw and break the chart. Corrected to data.0.<member> and {{#each data}}, matching the Handlebars reference panel in the chart editor.

JavaScript. The page says <script> tags are supported and shows a Chart.js CDN example. HTML charts render in a sandboxed iframe under a restrictive CSP, so scripts don't execute and external resources don't load. Replaced that section with an accurate description of what the sandbox allows, and pointed readers at built-in chart types or custom Vega-Lite specs for visualizations.

Also documents the rest of the template context (columns, rowCount, columnCount, pivot) and the available helpers, which previously only existed in the in-app panel.

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