Fix for #113, and related model downloading issues

[patrickjchen]

Fix GGUF package loading and downloading (#113)

A model directory only counted as a GGUF package when it held a file named
exactly model.gguf, so published packages under their release name fell
through to the safetensors source and demanded config files the GGUF never
ships — the "missing required json key: preprocess" in #113. Separately, the
WebUI downloader ignored strip_prefix, installing every audio-cpp/audio.cpp-gguf
package one directory too deep, so those downloads never validated.

Adds the self-contained vevo2_gguf package, and reworks the WebUI download
flow to state its cost (size, disk utilization, VRAM or system RAM share),
alarm past 75% disk / 80% device memory, and require an explicit confirmation
before fetching anything.

[0xShug0]

@patrickjchen Thanks! I'll take a look after merging the spec v1 branch, which should make both UI integration and model development easier. If your PR has any conflicts with the updated main, I'll resolve them directly.

[0xShug0]

Merged! Great extension for both the framework and UI.