feat: update checking via GitHub releases API

[JBolanle]

Summary

• Adds a check_for_update Rust command that silently checks the GitHub releases API (5s timeout, no CSP changes needed) and returns the latest version if newer than the current build
• Shows a persistent toast with a Download button (opens GitHub releases page) when an update is found; silent if already up to date
• Runs automatically on startup and manually via Help → Check for Updates… (new macOS menu item)

Test Plan

• Run npm test — 87 tests pass across 10 files
• Run cargo test from src-tauri/ — 38 tests pass
• Build and launch the app; confirm no update toast appears if already on latest
• Temporarily set CARGO_PKG_VERSION to 0.0.1 (or mock the API response) to verify the update toast appears with a Download button
• Click Help → Check for Updates… — confirm toast appears when update available, or "You're on the latest version" when not
• Click Download — confirm GitHub releases page opens in the browser

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