feat(updater): add Sparkle Stage 1 - minimal UpdateService, menu, and guards (opt-in)

[3manu31]

feat(updater): add Sparkle Stage 1 - minimal UpdateService (opt-in)

What does this PR do?

• Adds a minimal, opt-in in-app updater integration using Sparkle via SPM.
• Wires an UpdateService that initializes Sparkle's updater controller (without starting it automatically) and exposes checkForUpdates().
• Adds a Check for Updates command in the app menu to trigger the updater.
• Keeps a safe fallback: when no SUFeedURL is present the button opens the GitHub Releases page.

This is a conservative Stage 1 change intended only to add the UI hook and updater wiring so maintainers can review without needing CI secrets, appcast signing, or release-process changes.

Type of change

• Bug fix
• New feature (opt-in, non-invasive)
• Performance improvement
• UI/UX enhancement
• Documentation
• Other (describe)

Files changed

• project.yml — add Sparkle SPM package and target product dependency
• PureMac/Services/UpdateService.swift — new minimal updater service
• PureMac/PureMacApp.swift — add Updates command menu and Check for Updates button
• PureMac/ViewModels/AppState.swift — small concurrency fix (MainActor wrap for Logger call)
• PureMac.xcodeproj/project.pbxproj — Xcode project updated to include Sparkle/package entries

Why this change

• Stage 1: Introduce a safe, reviewable integration that does not require signing keys or CI changes.
• Keeps the codebase ready for a separate Stage 2 PR that would add appcast generation and signed updates.

Related issues

• Related: [Feature] Auto Updates #94

Testing done

• Built locally: xcodegen generate → open PureMac.xcodeproj → Build succeeded.
• Runtime: Tested on macOS Tahoe (26). Check for Updates opens Releases when SUFeedURL is not set; when SUFeedURL is present Sparkle's updater is started and checks for updates.

How to test locally

1. Ensure xcodegen is installed: brew install xcodegen
2. Generate project: xcodegen generate
3. Open in Xcode: open PureMac.xcodeproj
4. Build & run. From the app menu: Updates → Check for Updates.
   • With no SUFeedURL in the app's Info.plist the app opens the GitHub Releases page (safe fallback).
   • When maintainers add a feed URL + signed appcast, Sparkle will handle update checks.

Notes for reviewers

• This PR is intentionally minimal and conservative. It adds no CI changes, no signing keys, and no production appcast generation.
• If maintainers want Stage 2 (signed appcast, CI workflow updates), I can prepare a separate PR describing the required secrets and workflow changes.

AI-assistance disclosure

Drafted using Claude Code. Built the project locally to confirm my version compiles. I understand what the code does.