UI: TCP add-interface wizard should expose an opt-in "keep alive in background" toggle

[torlando-tech]

Follow-up from the dual-interface refactor (ea2e57e on feat/enable-tunnel-flip-flag).

Current behaviour

After the dual-interface change, the tunnel TunnelTCPInterface mirror is created automatically for the first enabled foreground TCP entity whenever Background Transport's TunnelManager reaches .connected. The user has no per-relay control: every TCP relay either gets a background mirror (if BT is on) or doesn't (if BT is off).

What it should look like

When the user adds or edits a TCP relay in the wizard (TCPClientWizard / TCPClientWizardViewModel), surface a checkbox along the lines of:

☐ Keep this connection alive in the background

When enabled, the Network Extension maintains a parallel TCP path to this relay even when Columba is suspended. Lets you receive a notification when a message arrives while the app is closed.

Trade-off: an extra TCP socket per relay (one app-process, one extension-process) — minimal battery cost on Wi-Fi, slightly more on cellular. Requires Background Transport (Settings → Background Transport) to be on.

When unchecked, this TCP entry stays foreground-only — no TunnelTCPInterface mirror is created for it.

Implementation pointers

• Adds a new field on TCPClientConfig (alsoTunneled: Bool, default true to match current behaviour) or on InterfaceEntity.
• AppServices.registerTunnelInterface() filters foreground entities by that field instead of mirroring all of them — combines with the multi-relay issue (#TBD when filed) into a single pass.
• Plain reactive wiring: editing the toggle re-runs reconcileTunnelInterface() (see related issue) so the change takes effect without a tunnel restart.

Why deferred

The architectural commit landed with a one-size-fits-all default. Per-relay control is a UX refinement, not a correctness gate.