flock-php install: report a declined permission prompt with an exit status, not text the apps match #25

What's wrong

The desktop apps tell a dismissed password or permission prompt apart from a real failure by matching the English error text that flock-php install prints:

• internal/cli/install_darwin.go:164 and internal/cli/install_windows.go:671 produce administrator permission was declined. feat(linux): a desktop app for Linux, with the tray, Settings, Dumps, setup and packaging #19 adds the same on Linux (internal/cli/install_linux.go).
• macos/Sources/FlockKit/Installation.swift:69 (permissionDeclined) matches it with output.contains(...).
• windows/FlockTray/SetupWindow.cs:115 matches it with output.Contains(...).
• feat(linux): a desktop app for Linux, with the tray, Settings, Dumps, setup and packaging #19's linux/app/frontend/setup.html matches it with output.includes(...).

cmd/flock/main.go exits with status 1 for every error, so the text is the only signal. If the message is reworded or translated, all three setup wizards quietly treat a declined prompt as a failure.

How it was found

@sineld raised this in the review of #19, on setup.html. I didn't change it there, because the same check is in the macOS and Windows apps too.

Suggested fix

• Give flock-php install a dedicated exit status when the prompt is declined, for example through an error type carrying an exit code that cmd/flock/main.go passes to os.Exit.
• Switch the three apps to check that status instead of the text.
• Keep the message as it is, for people reading the terminal.