dsh-file-upload: Adding Upload Button and Drag-and-Drop File Entry to DSH Sessions

dsh-file-upload is a community-developed plugin for the DSH web client, designed to simplify the upload and referencing of local files, images, and directories. The plugin enhances functionality by adding an upload button and supporting drag-and-drop features, archiving files into the DSH attachment library. For eligible images, it sends them to the official attachment rail and reuses the file_id from the DeepSeek Files API; other files are archived to specified directories, with a prefixed path text inserted into the draft. If the current model does not support images, images fall back to being represented as path text. The plugin supports recursive reading of dragged-in folders while preserving the original directory structure, offering settings options such as attachment directory, path prefix, official attachment routing toggle, and clipboard listener. File processing is limited by the DeepSeek API's 64 MB limit.