Fix Swift 6 actor-isolation warnings in model inits and services

- Message, Conversation, EmailLog: add nonisolated to inits — plain value
  types have no actor isolation, but the macOS 27 SDK was inferring it
- EncryptionService: replace lazy var encryptionKey (which mutates self and
  gets inferred as @MainActor) with an eagerly-initialized let in init()
- FileLogger: add nonisolated to shared, write, and minimumLevel so they
  are callable from nonisolated AppLogger methods without warnings
- LogLevel.<: add nonisolated to the Comparable conformance method

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

oAI/Services/EncryptionService.swift

@@ -31,12 +31,11 @@ import IOKit
 class EncryptionService {
     nonisolated static let shared = EncryptionService()
 
-    private let salt = "oAI-secure-storage-v1" // App-specific salt
-    private lazy var encryptionKey: SymmetricKey = {
-        deriveEncryptionKey()
-    }()
+    private let encryptionKey: SymmetricKey
 
-    private init() {}
+    private init() {
+        self.encryptionKey = Self.deriveEncryptionKey()
+    }
 
     // MARK: - Public Interface
 
@@ -73,19 +72,17 @@ class EncryptionService {
     // MARK: - Key Derivation
 
     /// Derive encryption key from machine-specific data
-    private func deriveEncryptionKey() -> SymmetricKey {
-        // Combine machine UUID + bundle ID + salt for key material
+    private static func deriveEncryptionKey() -> SymmetricKey {
         let machineUUID = getMachineUUID()
         let bundleID = Bundle.main.bundleIdentifier ?? "com.oai.oAI"
+        let salt = "oAI-secure-storage-v1"
         let keyMaterial = "\(machineUUID)-\(bundleID)-\(salt)"
-
-        // Hash to create consistent 256-bit key
         let hash = SHA256.hash(data: Data(keyMaterial.utf8))
         return SymmetricKey(data: hash)
     }
 
     /// Get machine-specific UUID (IOPlatformUUID)
-    private func getMachineUUID() -> String {
+    private static func getMachineUUID() -> String {
         // Get IOPlatformUUID from IOKit
         let platformExpert = IOServiceGetMatchingService(
             kIOMainPortDefault,