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/Models/Message.swift

@@ -66,7 +66,7 @@ struct Message: Identifiable, Codable, Equatable {
     // Reasoning/thinking content (not persisted — in-memory only)
     var thinkingContent: String? = nil
 
-    init(
+    nonisolated init(
         id: UUID = UUID(),
         role: MessageRole,
         content: String,