Reusable scanning architecture across Switchify PC #799

Implement the agreed reusable scanner architecture in two scoped PRs for Switchify PC (Mac and Windows).

• Unify scan navigation and timing behind a reusable engine #800 — Shared item engine and existing menu/keyboard migration. Draft PR Extract reusable menu and keyboard scanner core #803.
• Add scanner defaults and per-area customisation #801 — Shared defaults and per-area customisation. Stacked PR Add shared scanner defaults and per-area customisation #804, based on Extract reusable menu and keyboard scanner core #803.

Preserve point scanning, menu timing, keyboard/prediction behaviour, typed execution, cleanup and local/Remote parity. Native overlays remain nonactivating and click-through. No BLE protocol change, no Android work and no native-prediction spike.

VM-only native testing; restore settings and prior running states and leave the signed host Mac app untouched. Each stage requires independent latest-head review and passing frontend/Rust/CI checks. Merge and release require separate instructions.

App/settings-screen scanning (#802) is out of scope. Scanner customisation covers point scanning, menus and the keyboard only.