chore(build): migrate to SwiftPM + xTool, add AltStore/SideStore pipeline

- Replace XcodeGen project.yml with Package.swift (SwiftPM)
- Add xtool.yml for xTool cross-platform build configuration
- Add .gitea/workflows/release.yml — builds IPA on tagged release via
  self-hosted macOS runner, uploads to Gitea releases, auto-updates source
- Add altstore-source.json — dual-compatible with AltStore and SideStore
  (top-level downloadURL, ISO 8601 dates, SilverLABS branding)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

altstore-source.json

@@ -0,0 +1,24 @@
+{
+  "name": "SilverLABS",
+  "identifier": "uk.silverlabs.source",
+  "subtitle": "Privacy-first apps by SilverLABS",
+  "description": "SilverLABS app distribution — compatible with AltStore and SideStore.",
+  "iconURL": "https://git.silverlabs.uk/SilverLABS/SilverApple/raw/branch/main/SilverApple/Assets.xcassets/AppIcon.appiconset/Icon-60@2x.png",
+  "website": "https://silverlabs.uk",
+  "tintColor": "00BBFF",
+  "apps": [
+    {
+      "name": "SilverApple",
+      "bundleIdentifier": "uk.silverlabs.silverapple",
+      "developerName": "SilverLABS",
+      "downloadURL": "",
+      "subtitle": "Privacy companion for iOS",
+      "localizedDescription": "SilverApple integrates your device with the SilverLABS ecosystem — MDM enrollment, privacy hardening checks, and Passport authentication.",
+      "iconURL": "https://git.silverlabs.uk/SilverLABS/SilverApple/raw/branch/main/SilverApple/Assets.xcassets/AppIcon.appiconset/Icon-60@2x.png",
+      "tintColor": "00BBFF",
+      "category": "utilities",
+      "versions": []
+    }
+  ],
+  "news": []
+}