fix(macos-tray): download DMG matching host CPU architecture (#377)

The "Download update" menu item always opened the first darwin DMG
found in the GitHub release assets, which is the amd64 build on any
release (amd64 sorts before arm64). Apple Silicon users were pushed
toward the Intel installer.

Detect the host architecture via sysctl (handling Rosetta translation
so a translated process still gets offered the native arm64 DMG),
match release assets on both "darwin" and the arch token, and prefer
the signed/notarized "-installer.dmg" variant when present.

Co-authored-by: Claude Code <noreply@anthropic.com>