dsh-attach-picker: Adding a System-Level Image Picker Button to the DSH Web Input Bar

`dsh-attach-picker` is a zero-dependency plugin for DeepSeek Harness (DSH) Web, designed to provide a more intuitive image upload experience on mobile devices and in specific scenarios. It adds an image button to the input bar toolbar, invokes the native file picker with multi-select support, and strictly validates image formats and size limits. The plugin reuses the existing pipeline, automatically adapts to DSH versions, and effectively resolves the inconvenience of relying solely on drag-and-drop or pasting.