Audit remediation

Implements audit PR-06 for actionable platform diagnostics and a safe restart flow.

• discovery failures are returned as typed diagnostics rather than collapsed into “not installed”
• Windows asks the selected target to close normally and waits before any escalation
• an explicit second confirmation is required before the force-close path; cancellation preserves the current target and theme state
• macOS persistence hosts are restricted to /Applications and the current user’s ~/Applications

Verification

• npm run desktop:check (30 Rust tests)
• npm run test:source
• node --check apps/theme-manager/src/renderer/app.js
• node --check apps/theme-manager/src/renderer/bridge.js
• git diff --check

Independent acceptance pass

Pass with risks: source and Rust acceptance checks cover typed discovery failures, normal-close-first behavior, escalation authorization, cancellation state, and macOS host validation. A real isolated Tauri/UI smoke test against Stable/Beta and a live Windows graceful/force-close lifecycle are intentionally not run here, so those remain required before release.

Closes #9