Allow importing account-less legacy instances as sources

Legacy v1 instances without an account had no import path after the
switch to explicit account review. Surface them in a separate "Sources"
section of the legacy import view where they can be added with a single
tap (no sign-in) or removed, mirroring the accounts flow. Instances tied
to an account stay in the Accounts section and are not duplicated.

Detection now gates the entry points and first-launch prompt on any
legacy data (accounts or sources), not accounts alone.

Yattee/Services/Migration/LegacyDataModels.swift

@@ -173,3 +173,35 @@ struct LegacyAccountImportItem: Identifiable, Sendable {
         return url.host ?? url.absoluteString
     }
 }
+
+// MARK: - Legacy Instance Import Item
+
+/// Represents a legacy instance (source) that can be re-created without signing in.
+/// Used for v1 instances that have no associated account.
+struct LegacyInstanceImportItem: Identifiable, Sendable {
+    /// The original v1 instance ID.
+    let legacyInstanceID: String
+
+    /// The type of instance.
+    let instanceType: InstanceType
+
+    /// The instance URL.
+    let url: URL
+
+    /// User-defined instance name, if any.
+    let instanceName: String?
+
+    /// Whether this instance proxies videos.
+    let proxiesVideos: Bool
+
+    /// Stable identifier for SwiftUI lists.
+    var id: String { legacyInstanceID }
+
+    /// Display name for the source row.
+    var instanceDisplayName: String {
+        if let instanceName, !instanceName.isEmpty {
+            return instanceName
+        }
+        return url.host ?? url.absoluteString
+    }
+}