refactor: shared useReconnect hook for the handoff/reconnect overlays

[KrasimirKralev]

Deferred /simplify follow-ups from #168, carried in #167 (whose e2e/floor work landed in #189):

• A shared useReconnect({ probe, graceMs, onReady }) controller hook to collapse the duplicated grace-timer/probe-loop engine across WifiHandoffOverlay, CredentialsHandoffOverlay, ReconnectingOverlay (and the 4th copy in SettingsApp.tsx).
• CredentialsStep handoff-decision dedup.
• getEthernetStatus / start-ap.sh nmcli micro-optimizations.

Pure refactor — no behavior change intended. The handoff paths now have e2e (setup-wifi-handoff.spec.ts) guarding the WiFi flow, which makes this refactor safer than when it was deferred. Touches setup-critical reconnect logic, so validate on a device before merging.

[KrasimirKralev]

Triage: enhancement (refactor, no user-visible change). Extracting the shared useReconnect hook is good hygiene but low urgency — good candidate for a slow week or a first-time contributor.