[2.8.0] Select signed plaintext transport for licensed remote Admin

[RCGV1]

Description

Implement the Android/Desktop client transport selection required by meshtastic/design#123 and meshtastic/firmware#10972.

When the connected local radio is licensed, every remote Admin getter, session request, and mutation must use channel 0 with pki_encrypted=false and an empty packet public key. Firmware then signs and authorizes the plaintext packet using existing XEdDSA. Normal-mode remote Admin behavior remains unchanged.

Scope

• Update the central CommandSenderImpl Admin channel/packet selection, not each Admin command individually.
• Determine licensed mode from the connected local node's User.is_licensed; never infer it from the destination callsign/licensed bit.
• Apply the same policy to sendAdmin, sendAdminAwait, metadata/session refresh, and all flows that use buildAdminPacket.
• Preserve self Admin behavior.
• Preserve existing normal-mode PKI selection when local and destination keys are available.
• Do not display an encryption lock for licensed Admin; use verified-signature semantics only after firmware verification.

Acceptance criteria

• Licensed local → remote Admin uses channel 0, pki_encrypted=false, and empty public key even when both nodes have keys.
• Non-licensed local with both keys retains existing PKI Admin behavior.
• Self Admin remains unchanged.
• Missing local owner state follows existing compatibility behavior rather than assuming licensed mode.
• Tests cover getter, session request, mutation, normal-mode regression, self, and unknown local mode.
• PR remains draft until paired two-radio validation with firmware#10972 proves plaintext readable Admin request/reply and successful signed authorization.

Dependencies

• Design: [FOLLOW-UP] [ALIGNMENT]: Signed plaintext remote administration in licensed mode design#123
• Firmware: Authorize signed plaintext remote admin in licensed mode firmware#10972
• Licensed signing: Sign plaintext packets in licensed mode firmware#10969

[RCGV1]

Implementation is in progress by @RCGV1 as a separate draft follow-up to firmware#10972. GitHub did not retain self-assignment for this contributor account; please coordinate here before starting duplicate work.