Refresh the installed engine when the app launches

[HexCodeYT]

What changed

• Reinstalls and restarts the bundled PlainLink engine on every normal app launch.
• Ensures drag-and-drop upgrades replace an older CLI under Application Support.
• Records an explicit Pause Cleaning choice so later launches do not unexpectedly restart a watcher the user paused.
• Keeps first-run guidance separate from engine synchronization.
• Adds Preview 4 release notes focused on upgrade behavior.

Root cause

The menu app installed the embedded CLI only on its first automatic-enable attempt. Replacing PlainLink.app with a newer preview did not necessarily overwrite the older watcher binary already installed under ~/Library/Application Support/PlainLink/bin, so the menu app could appear current while clipboard cleaning still used old recognition logic.

Verification

• cargo fmt --check
• cargo test
• cargo clippy --all-targets -- -D warnings
• scripts/test-macos-app.sh
• PLAINLINK_RELEASE_VERSION=v0.1.0-preview.4 scripts/package-macos-app.sh
• Preview 4 checksum verification

All checks passed on macOS arm64.