ggml-webgpu: makes the flash attn vec path subgroup-aware (#23040)

* ggml-webgpu: makes the flash attn vec path compile and size its split/reduce work from the device’s reported subgroup range instead of assuming 32 subgroup size.

* ggml-webgpu: remove the extra max_wg_size >= max_subgroup_size guard. Remove hardcoded 32 when determine the value of reduce_wg_size and vec_nwg_cap