[js/webgpu] Add DFT operator

[nicholascelestin]

Description

Implements the DFT operator (opset 17–20) for the WebGPU (JSEP) execution provider: a batched shared-memory mixed-radix (2/3/4/5) Stockham FFT, with an O(N^2) direct-DFT fallback for non-5-smooth lengths. Handles real/complex input, forward/inverse (1/N-normalized), one-sided RFFT and one-sided-inverse IRFFT, and the opset-20 dft_length/axis tensor inputs — matching the semantics of the CPU kernel in core/providers/cpu/signal/dft.cc.

Motivation and Context

The WebGPU EP had no DFT/FFT kernel, so spectral and audio-preprocessing models fall back to a slow path (#20997). This adds it to the JSEP EP. The op test suite was extended to cover the WebGPU backend; I also verified the kernel against ORT's CPU DFT on real WebGPU across the forward, inverse, RFFT, and IRFFT paths. As with other onnxruntime-web op PRs, CI is the source of truth for the full web build.

Closes #20997

[nicholascelestin]

@microsoft-github-policy-service agree

[nicholascelestin]

@guschmue , know this might take a bit, but was hoping a maintainer could kick off the CI checks, so we've got a place to start?

[nicholascelestin]

@qjia7, just pinging to try and unblock this, since it currently has no reviewers and CI hasn't run yet.

[qjia7]

Thanks for the contribution. Could you also add the WebGPU implementation under the WebGPU provider directory? JSEP will be deprecated soon and replaced by the WASM + WebGPU architecture.

[nicholascelestin]

Thanks for the contribution. Could you also add the WebGPU implementation under the WebGPU provider directory? JSEP will be deprecated soon and replaced by the WASM + WebGPU architecture.

This good?