[2/6] Check app releases from Settings

[BinaryBearsLLC]

Part 2 of 6 — depends on #169. Please keep this PR in draft until the preceding parts are merged. Its current Files changed tab includes those earlier changes.

Review and merge order: #169 → #170 → #171 → #172 → #173 → #174.

Before merging: refresh this branch against the updated main, retaining only this part’s commit. If earlier parts were squash-merged or rebased, reapply only the linked commit onto updated main; do not merge the cumulative diff as-is.

Review this part’s commit.

What does this PR do?

Adds an on-demand app update check to Settings, alongside the installed version and build number.

• Checks official GitHub app releases and excludes CLI releases, drafts and prereleases.
• Shows update availability or a clear error, with download links and Homebrew instructions.
• Reuses the main window when checking from the Updates menu. No update is installed automatically.

Testing

• Tested on macOS Ventura 13.6 (22G120, Apple Silicon VM).
• Host XCTest suite and universal arm64/x86_64 build on macOS 26.6.2.

106 XCTest tests passed, including version ordering, release filtering and request failures; universal build succeeded on macOS 26.6.2 (Apple Silicon). Tested on macOS Ventura 13.6 (22G120, Apple Silicon VM). GUI checks passed for a live GitHub release check, DMG/Homebrew instructions, copied command and menu routing.

Screenshots