In-app auto-updater with download and install

[darktorres]

Description

Extend the existing update notification (which checks GitHub Releases) into a full auto-updater that can download and install updates without the user leaving the app.

Current State

The update checker exists (74d22b32a) and shows a dialog when a new version is available with a link to the release page. It does not download or install updates.

Acceptance Criteria

• Non-intrusive notification when an update is available
• One-click download and install (restart required)
• Rollback on failed update
• User can disable auto-check in settings
• Works on all three platforms

Implementation Guide

1. Download

• Extend UpdateChecker to download the release asset (AppImage/ZIP/DMG) from the GitHub Releases API
• Download to a temp directory with progress reporting
• Verify checksum if available in the release notes

2. Platform-specific installation

• Linux AppImage: Replace the current AppImage file, re-launch
• Windows: Download the installer/ZIP, launch installer, quit current instance
• macOS: Download DMG, mount, replace .app bundle (or use Sparkle framework)

3. Rollback

• Before replacing, backup the current binary
• If the new version fails to launch, restore the backup
• Store backup path in settings for cleanup on next successful launch

4. UI

• Extend the existing update dialog with "Download & Install" button
• Show download progress bar
• Add "Check for Updates" in Help menu (already exists — add download action)

Key files

• App/Core/UpdateChecker.h/.cpp — existing update check logic
• App/UI/MainWindow.cpp — update dialog
• App/Core/Settings.h/.cpp — auto-update preference