fix: prevent unicode corruption from thread-unsafe OCR mutation

[Project-A-E-G-I-S]

Summary

Fixes a thread safety violation where VNRecognizeTextRequest's completion handler mutated a SwiftData @Model property from a background thread, gradually corrupting the ModelContext's internal state and causing random unicode corruption across all clipboard items.

Root Cause

In HistoryItem.generateTitle() (Maccy/Models/HistoryItem.swift:88-94), when the copied item is an image, a Task fires performTextRecognition(). Since generateTitle() is not @MainActor, this Task runs on the default global concurrent executor.

VNImageRequestHandler.perform() is synchronous (Maccy/Models/HistoryItem.swift:226) — the recognizeTextHandler completion handler executes on the same background thread. It directly set self.title on a SwiftData @Model object (line 241), which is only safe on the @MainActor.

This thread safety violation corrupts the shared ModelContext's internal snapshot and change tracking state. Over days of use, the corruption spreads to all loaded objects — not just images — manifesting as random unicode characters in clipboard text.

Quitting the app clears the in-memory context, which is why it temporarily 'fixes' itself on relaunch.

Fix

Capture the recognized text on the background thread (safe), then dispatch the SwiftData model mutation to @MainActor:

let text = recognizedStrings.joined(separator: "\n")
Task { @MainActor in
    self.title = text
}

Testing

• Existing HistoryItemTests continue to pass
• The OCR path is exercised whenever an image item's title is generated

Closes #1374