fix FK constraint: skip duplicate messages before thread reconstruction

- insertMessages checks (mailboxId, uid) existence before INSERT, returns
  only actually inserted records
- syncMailbox only runs ThreadReconstructor on newly inserted messages,
  preventing FK violations from stale UUIDs referencing ignored records
- improve error logging to show full error description

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Apps/MagnumOpus/ViewModels/MailViewModel.swift

@@ -207,10 +207,10 @@ final class MailViewModel {
 			try await coordinator.syncNow()
 			syncState = coordinator.syncState
 		} catch {
-			let desc = "\(type(of: error)): \(error.localizedDescription)"
+			let desc = "\(error)"
 			print("[MailViewModel] syncNow failed: \(desc)")
-			errorMessage = desc
-			syncState = .error(desc)
+			errorMessage = error.localizedDescription
+			syncState = .error(error.localizedDescription)
 		}
 	}