Improve first-run diagnostics

[man612]

Problem

First-time users may not know whether USB debugging, ADB authorization, bundled binaries, or VPN permission is blocking the connection.

Proposed work

• Add a lightweight diagnostics checklist in onboarding or dashboard state.
• Report missing runtime files clearly.
• Show next-step guidance before starting the engine.
• Keep messages short and beginner-friendly.

Verification

• Test with no device connected.
• Test with unauthorized device.
• Test with missing Gnirehtet/ADB runtime files in a dev copy.