Privileged backend: QProcess + pkexec wrapper and machine-readable CLI output

[vhrabar]

Description

Single audited privilege boundary. The CLI already resolves the real user from PKEXEC_UID, so pkexec secureEye -U <user> … is the transport. In C++ this means an async QProcess wrapper plus strict parsing,which requires the CLI to emit structured output.

Tasks

• GuiBackend class (QObject): async run(command, args) via QProcess, signals for finished/failed/cancelled; never blocks the UI thread
• Add/verify --plain or JSON output on the CLI commands the GUI consumes (list has --plain; audit add, remove, clear, disable, set, version), parse with QJsonDocument, never scrape human-readable text
• Ship polkit policy com.github.vhrabar.secureeye.policy with separate action IDs for read vs write (…manage-models, …modify-config, …control-service)
• Map pkexec exit codes (126 dismissed, 127 not authorized) and CLI non-zero exits to distinct user-visible errors
• Refuse to start if launched as root (check geteuid() at startup, explain why)

Acceptance criteria

• Cancelling the polkit dialog leaves every view consistent
• All CLI interaction goes through GuiBackend; no raw QProcess calls elsewhere
• Policy file installed by meson and listed in packaging