Fix startup crash in sideloaded builds without iCloud entitlements

Sideloading tools re-sign the app with a team that cannot register the
iCloud.stream.yattee.app container, and CKContainer(identifier:) fatally
traps when the entitlement is missing. Detect availability by parsing
the embedded provisioning profile, skip creating any CloudKit objects
when the entitlement is absent, and show an explanation in iCloud
settings instead of the sync toggle.

Yattee/Services/CloudKit/CloudKitAvailability.swift

@@ -0,0 +1,51 @@
+//
+//  CloudKitAvailability.swift
+//  Yattee
+//
+//  Detects whether the app was signed with the iCloud container entitlement.
+//
+
+import Foundation
+
+/// Sideloaded installs (AltStore, SideStore, Sideloadly, …) are re-signed by a
+/// team that cannot register `iCloud.stream.yattee.app` (container IDs are
+/// globally unique), so the iCloud entitlements are stripped or remapped.
+/// `CKContainer(identifier:)` fatally traps in that state, so the container
+/// must never be created when the entitlement is missing.
+enum CloudKitAvailability {
+    /// Whether CloudKit APIs may be used in this installation.
+    ///
+    /// Determined by parsing the embedded provisioning profile. Builds without
+    /// an embedded profile (App Store, TestFlight, simulator) always carry the
+    /// correct entitlements and are treated as available.
+    static let isAvailable: Bool = {
+        guard let entitlements = embeddedProvisioningEntitlements() else {
+            return true
+        }
+        let containers = entitlements["com.apple.developer.icloud-container-identifiers"] as? [String] ?? []
+        return containers.contains(AppIdentifiers.iCloudContainer)
+    }()
+
+    /// Extracts the entitlements dictionary from the embedded provisioning
+    /// profile — a CMS blob wrapping an XML property list.
+    private static func embeddedProvisioningEntitlements() -> [String: Any]? {
+        #if os(macOS)
+        let url = Bundle.main.bundleURL.appendingPathComponent("Contents/embedded.provisionprofile")
+        #else
+        guard let path = Bundle.main.path(forResource: "embedded", ofType: "mobileprovision") else { return nil }
+        let url = URL(fileURLWithPath: path)
+        #endif
+
+        guard let data = try? Data(contentsOf: url),
+              let start = data.range(of: Data("<?xml".utf8)),
+              let end = data.range(of: Data("</plist>".utf8), in: start.lowerBound..<data.endIndex),
+              let plist = try? PropertyListSerialization.propertyList(
+                  from: data.subdata(in: start.lowerBound..<end.upperBound),
+                  format: nil
+              ),
+              let profile = plist as? [String: Any]
+        else { return nil }
+
+        return profile["Entitlements"] as? [String: Any]
+    }
+}