Introduced a resizable splitter for the PDF thumbnail sidebar, allowing users to dynamically adjust its width. Added `_thumbnailWidth` property with min/max constraints and implemented mouse event handlers (`OnSplitterMouseDown`, `OnSplitterMouseMove`, `OnSplitterMouseUp`) to manage resizing.
Integrated JavaScript interop to attach/detach resize event listeners and save user preferences to `localStorage`. Updated `pdf-viewer.js` to handle resizing state and cleanup. Styled the splitter in `envelope-viewer.css` with hover/active states and ensured smooth interaction.
Persisted thumbnail width across sessions and added error handling for `localStorage`. Enhanced user experience with intuitive resizing and improved UI flexibility.
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