<details open> webgpu : add CONV_2D_DW (depthwise conv2d) kernel (#25847) * webgpu : add CONV_2D_DW (depthwise conv2d) kernel Implement GGML_OP_CONV_2D_DW for the WebGPU backend, ported from the Vulkan backend's conv2d_dw.comp. Assisted-by: Claude Opus-4.8 * Remove unnecessary comments in webgp