sidebar startup: suppress stale engine error state on fresh launch

[PowerBeef]

A fresh launch should only surface current startup and runtime state, not stale prior error state.

Source report: #22

Problem

The sidebar can show a stale engine error card on fresh launch before the user performs any new interaction.

Evidence

• April 18, 2026 validation report: Dev validation report: April 18, 2026 #22
• Manual validation saw a stale empty-custom-prompt style engine error surface on fresh launch before any new action
• The report treated this as user-visible but not yet a confirmed packaged/release regression

Current ownership

• Sources/Services/AppEngineSelection.swift
• Sources/QwenVoiceNative/XPCNativeEngineClient.swift
• Sources/Views/Components/SidebarStatusView.swift

Acceptance

A fresh launch shows only current startup/runtime state, and the sidebar does not surface stale generation errors unless a new action actually triggers one.

Focused rerun

• python3 scripts/harness.py validate
• Scoped Computer Use pass for fresh launch and sidebar status