[Feature request]: Allow adding local GGUFs from disk

**What problem would this solve?**
It is incredibly wasteful to re-download gigabytes of data already present on the machine.
As well as it being near impossible on low-bandwidth networks.

**What would a good outcome look like?**
Being able to load GGUFs without having to _specifically_ download them through the app.

**Possible approaches**
Add a "Add local..." button to Local Inference to open a file chooser for GGUF.

**Additional context**
N/A

- [x] I have verified this does not duplicate an existing feature request

Do not begin implementation until the issue reaches **Ready** on the [Goose Issues board](https://github.com/orgs/aaif-goose/projects/1).

Look for the Local Inference UI component and the model loading logic. The feature involves adding a file chooser dialog and integrating local GGUF file loading into the existing model management flow. Check how models are currently downloaded and instantiated to understand the required changes.

Written by the indexing model from the issue text.