feat: retarget the auto-updater from Gitea to GitHub

The source repo moved to github.com/debont80/AeroFetch; the in-app
updater and release links still pointed at the old self-hosted Gitea
instance. Repoint RELEASE_API at the GitHub REST API, replace the
single-host download trust check with an allowlist (github.com plus
its release-asset CDN hosts, which GitHub 302s downloads through —
mirrors the existing FFMPEG_TRUSTED_HOSTS pattern), and add the
User-Agent header GitHub's API requires.

No bridge: installs on a Gitea-pointed build (<=0.7.3) won't
auto-discover releases published to GitHub and need one manual
download, matching the precedent set by the earlier releases-repo
migration.

README.md

@@ -50,7 +50,7 @@ It ships as a **regular installer** *and* a **single portable `.exe`** (no admin
 
 ## Download & install
 
-Grab the latest build from the [**Releases**](https://gitea.netbird.zimspace.uk:5938/debont80/AeroFetch/releases) page:
+Grab the latest build from the [**Releases**](https://github.com/debont80/AeroFetch/releases) page:
 
 - **`AeroFetch Setup <version>.exe`** — standard installer. Installs per-user (no admin), adds a Start-menu shortcut, and auto-updates.
 - **`AeroFetch-<version>-portable.exe`** — single self-contained executable. No install, no admin; keeps its data in an `AeroFetch-data` folder beside the `.exe`. Ideal for locked-down or shared PCs.
@@ -64,7 +64,7 @@ Each release also publishes a `.sha256` next to every executable so you can veri
 Requires **Node.js 24.x**.
 
 ```bash
-git clone https://gitea.netbird.zimspace.uk:5938/debont80/AeroFetch.git
+git clone https://github.com/debont80/AeroFetch.git
 cd AeroFetch
 npm install