Add a coin-control path so the user can spend specific coins:

- send_ark_selected command — wraps the SDK's send_vtxo_selection to  spend exactly the named outpoints, skipping automatic selection. Ark-only; the SDK has no onchain-offboard selection variant.
- Coins screen "Select" mode — pick VTXOs, then a drawer to send them to an Ark address

- Add an exponential-backoff retry layer for transient esplora transport errors and a socket timeout so a stalled connection can't hang a syn cycle.
- Gate sync-failure UI toasts behind a sustained failure streak so a lone device-sleep failure stays silent while real outages still get reported.
- Make the Esplora explorer presets network-aware.