HubApi default downloadBase triggers macOS TCC "Documents folder" prompt for apps that never use the Hub

Hi, thanks for publishing this library!

Here's a note from Claude. A patch from Claude did fix this issue for my setup.

## Problem

`HubApi.init()` defaults `downloadBase` to `~/Documents/huggingface` when no explicit path is provided:

```swift
// Sources/Hub/HubApi.swift:163-164
let documents = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first!
self.downloadBase = documents.appending(component: "huggingface")
```

On macOS Sequoia+, resolving `.documentDirectory` triggers a TCC consent prompt: *"App would like to access files in your Documents folder."*

This affects any app that links the `Tokenizers` module — even if the app never downloads from the Hub. The `static let shared = HubApi()` singleton is lazily initialized when Swift evaluates default parameters like `hubApi: HubApi = .shared` in `AutoTokenizer.from(modelFolder:)`, which is enough to trigger the prompt.

## Impact

- Apps that only use `Tokenizers` for local model loading get an unexpected Documents folder permission prompt
- Users may deny the prompt, causing confusion
- The `downloadBase` directory (`~/Documents/huggingface`) is never actually used in these cases

## Suggested fix

Change the default `downloadBase` from `.documentDirectory` to a non-TCC-protected location. Options:

1. **`~/.cache/huggingface`** — matches Python `huggingface_hub` convention, matches `swift-huggingface`'s `CacheLocationProvider.defaultCacheDirectory()` on non-sandboxed macOS
2. **`.applicationSupportDirectory`** — standard macOS app data location
3. **`.cachesDirectory` (`~/Library/Caches/`)** — simplest change but subject to system purging

Option 1 seems most consistent with the ecosystem. Any change would need migration consideration for existing users who have snapshots in `~/Documents/huggingface/`.

## Reproduction

1. Create a macOS app that depends on `swift-transformers` (Tokenizers product)
2. Call `AutoTokenizer.from(modelFolder: someLocalPath)` without passing `hubApi:`
3. On macOS Sequoia+, the app shows a "Documents folder" TCC prompt on first run