Add optional launch_cmd to auto-start Ardop/VARA/AGWPE TNC daemons

[mwthomson]

Addresses #537.

Adds an optional launch_cmd setting to Pat's config for the four transports that depend on an external TNC daemon: Ardop, VaraHF, VaraFM, and AX.25/AGWPE. When set, Pat spawns that daemon itself the first time the transport is used, instead of requiring the user to start it by hand first.

"ardop": {
  "addr": "localhost:8515",
  "launch_cmd": {"path": "ardopcf", "args": ["--webgui", "8514"]}
}

Behavior, consistent across all four transports:

• Skips the launch if the daemon is already reachable (no duplicate spawns, even under concurrent connect/listen).
• On outgoing connect, waits up to 5s (polling every 500ms) for a freshly-launched daemon to come up before giving up.
• On incoming listen, relies on the existing ListenerHub retry loop rather than blocking.
• Relaunches automatically if the daemon crashes or is killed mid-session.
• Terminates any process it spawned when Pat itself exits (App.Close()) — no orphaned daemons.
• Logs a terse Pat failed to launch <transport> on failure; full error only at debug level.
• Strictly opt-in — leaving launch_cmd unset is a complete no-op, existing behavior unchanged.

This directly solves the underlying need in #537 (bringing up ardopcf's webgui alongside Pat) without Pat embedding or knowing anything about that UI — it just launches whatever command is configured.

Test plan

• go build ./..., go vet ./... clean
• go test ./... passes
• Unit tests cover: skip-when-reachable, launch-then-connect within poll budget, launch failure (bad path / never reachable) with exact log message, no duplicate spawn under concurrent Connect+Listen (-race), relaunch after crash, App.Close() terminates spawned processes
• Manually tested against a running AGWPE/Direwolf setup

[mwthomson]

General cleanup and fixes for #547, #548, and #549 now rolled into this PR. Ready for review