[FEAT] 'Exchange Position' with selectable channel index (instead of always Primary)

[seh-len]

In Meshtastic, position precision is explicitly a channel-specific setting.
(meshtastic.org)

In MeshMonitor, 'Exchange Position' currently (as in many UIs) effectively always triggers via the Primary Channel (index 0). This is perfectly fine as a default - but in practice it causes a problem:

• Many users keep Primary (LongFast/public) with coarse or disabled position sharing/precision (privacy).
• More precise position sharing is configured on a secondary private channel (e.g. index 1).
• If 'Exchange Position' always uses Primary, you may only get 'too imprecise' / 'no position' back, even though both devices could share precise position data on a private channel.

Meshtastic already supports selecting the channel index in the CLI (e.g. --request-position ... --ch-index N).
(meshtastic.org)

And there is already an open issue on this topic in the Meshtastic Android project.
(github.com)

Requested Behavior
For 'Exchange Position', it should be possible to select the channel index, so the request is sent on the desired channel and the recipient can respond according to its channel-specific precision configuration.

Proposed UI/UX
Directly next to the existing 'Exchange Position' button:

• small dropdown arrow / split-button
• selection: Primary (0) as the default via main button + additional channels (1..7) via dropdown