preview: improve sidebar layout and screenshot export

preview/lib/html2canvas/LICENSE

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+Copyright (c) 2012 Niklas von Hertzen
+
+Permission is hereby granted, free of charge, to any person
+obtaining a copy of this software and associated documentation
+files (the "Software"), to deal in the Software without
+restriction, including without limitation the rights to use,
+copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the
+Software is furnished to do so, subject to the following
+conditions:
+
+The above copyright notice and this permission notice shall be
+included in all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
+OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
+HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
+WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
+OTHER DEALINGS IN THE SOFTWARE.

preview/lib/html2canvas/README.md

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+# html2canvas
+
+Version: **1.4.1**, distributed under the [MIT license](LICENSE).
+
+The preview uses this local copy to export the visible email container as PNG without a CDN connection. Release archives include the library and its license.
+
+- Project: https://github.com/niklasvh/html2canvas
+- Distribution: https://cdn.jsdelivr.net/npm/html2canvas@1.4.1/dist/html2canvas.min.js
+- SHA-256: `e87e550794322e574a1fda0c1549a3c70dae5a93d9113417a429016838eab8cb`
+
+Capture uses DOM rendering, so CSS support and cross-origin images follow [html2canvas's documented limitations](https://html2canvas.hertzen.com/documentation). The iframe permits same-origin DOM access while keeping scripts disabled. Keep the version, license and checksum together when updating the library.

preview/lib/html2canvas/html2canvas.min.js

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+/*!
+ * html2canvas 1.4.1 <https://html2canvas.hertzen.com>
+ * Copyright (c) 2022 Niklas von Hertzen <https://hertzen.com>
+ * Released under MIT License
+ */
+!function(A,e){"object"==typeof exports&&"undefined"!=typeof module?module.exports=e():"function"==typeof define&&define.amd?define(e):(A="undefined"!=typeof globalThis?globalThis:A||self).html2canvas=e()}(this,function(){"use strict";
+/*! *****************************************************************************
+    Copyright (c) Microsoft Corporation.
+
+    Permission to use, copy, modify, and/or distribute this software for any
+    purpose with or without fee is hereby granted.
+
+    THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH
+    REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
+    AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT,
+    INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
+    LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR
+    OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
+    PERFORMANCE OF THIS SOFTWARE.
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