dsh-file-drop-eac: Drag and drop files/folders directly into an EAC conversation

`dsh-file-drop-eac` is a plugin in the Deepseek Harness EAC environment that replaces the legacy `dsh-file-drop` plugin. It resolves an issue in the EAC environment where the old plugin could conflict with the visual bridge by injecting duplicate images, and it adds support for folders. Implemented entirely on the client side, the plugin applies different strategies based on file type: text and code files are injected directly up to a 256KB limit; binary and large files are injected as paths for the agent to read; images are fully handled by the existing visual bridge to avoid conflicts; and folders are represented with path hints. The plugin supports retrieving real file paths in the Electron desktop version, while in the Web version it is constrained by browser security policies, and it uses classic-script mode for deep integration with EAC.