[FEATURE REQUEST] Add Evergreen as a package source via the Evergreen API

### Please confirm these before moving forward.

- [x] I have searched for my feature proposal and have not found a work-in-progress/duplicate/resolved/discarded issue.
- [x] This proposal is a completely new feature. If you want to suggest an improvement or an enhancement, please use [this template](https://github.com/Devolutions/UniGetUI/issues/new?labels=enhancement&projects=&template=enhancement-improvement.yml&title=%5BENHANCEMENT%5D+%28Enter+your+description+here%29).

### Describe the new feature

Add **Evergreen** as an optional UniGetUI package source, using the public Evergreen REST API (same data as `Get-EvergreenApp`).

- Product: https://eucpilots.com/evergreen/
- API docs: https://eucpilots.com/evergreen/api
- Base URL: `https://evergreen-api.stealthpuppy.com`
- Catalog today: **568 apps**, refreshed about every 8 hours from **official vendor** update sources (not a community feed).

Evergreen is not Chocolatey/Winget. It does not own an install/uninstall database. Each app is a list of **current vendor installers**: version, architecture, channel, type (`msi`/`exe`), SHA256, size, and a direct `URI`.

**Suggested UX (optional source, off by default)**

1. Discover: `GET /apps` → package list (`Name`, `Application`, `Link`).
2. Details / install / update: `GET /app/{appName}` → pick one variant, download `URI`, run the installer (same path UniGetUI already uses for download-then-install).
3. Default variant: current OS architecture, then `Channel=Stable` / `Release=Enterprise` when those fields exist. Let the user override channel/arch/type in package options.
4. Integrity: verify SHA256 when the payload includes it.

Example (Microsoft Edge Enterprise x64 Stable):

```http
GET https://evergreen-api.stealthpuppy.com/app/MicrosoftEdge
User-Agent: UniGetUI/{version}
```

Returns rows such as `Version`, `Channel`, `Release`, `Architecture`, `Type`, `SHA256`, `URI`.

**API constraints UniGetUI would need to respect**

- A **custom User-Agent is required**. Default PowerShell/curl/browser UAs are blocked. Please send something identifiable, e.g. `UniGetUI/3.x` (not a generic `Mozilla/...`).
- The API runs on Cloudflare Workers’ free tier (**100,000 requests/day**) and is documented as **for development, not distributed endpoints**. A built-in UniGetUI manager that polled every app on every refresh would get UniGetUI blocked and starve other users.
- Cache `GET /apps` and `GET /app/{name}` locally for **at least 8 hours** (matches their refresh).
- Do not enumerate `/app/{name}` for the full catalog on startup; fetch per package on search/details/update check.
- Optional fallback: call the Evergreen PowerShell module (`Get-EvergreenApp`) so each machine hits **vendor APIs directly** instead of the shared Worker. That is the documented production-safe path; the REST API is the easy integration.
- Variant explosion: some apps have a handful of rows (`VMwareTools` = 2); others have thousands (`MozillaFirefox` was 3000+). Filtering by arch/channel/locale is required or the UI will be unusable.
- Uninstall / “installed packages” will be weaker than Winget/Scoop. Detect via ARP / existing UniGetUI installed-package matching, or treat Evergreen as discover + download + install + version-compare only.

I searched `evergreen` on this issue tracker and found no duplicate.

### Describe how this new feature could help users

Winget (and the other managers) are great for many desktop apps, but they often lag or omit **enterprise SKUs and vendor-direct installers**: Edge Enterprise MSI, Adobe Acrobat/Reader channels, VMware Tools, FSLogix, Citrix Workspace, etc. Evergreen’s point is exactly that gap: “current version + official download URL from the vendor,” with hashes.

For IT / MSP / image-building users who already live in UniGetUI, this would mean:

- One UI to find the **vendor-current** build instead of hunting vendor sites or maintaining one-off scripts.
- Safer downloads (`SHA256` on most payloads) than an unverified web link.
- A complement to Winget, not a replacement: keep Winget/Scoop/Chocolatey for apps they cover well; use Evergreen when you need the official current installer.

Happy to iterate on scope (discover-only vs full install/update) if a thin first slice is more realistic given the shared API limits.

Start by reading UniGetUI's existing package-source integrations and the Evergreen API documentation, especially the /apps and /app/{appName} endpoints. Done should include an optional, off-by-default Evergreen source with per-package caching, filtered variant selection, a required custom User-Agent, SHA256 verification when available, and the agreed install or discovery scope.

Written by the indexing model from the issue text.