Separate Companion message history from pending inbox

Default to retained history with a separate pending count, and add persistent display.inbox CLI modes for history, pending, and unread. Preserve browsing while clients are connected and show the active USB or Bluetooth connection.

Fixes #5

docs/cli_command_availability.md

@@ -84,6 +84,7 @@ over the normal binary USB, BLE, or TCP connection:
 | [`get/set bluetooth.name`](cli_commands.md#view-or-change-the-independent-bluetooth-name-companion) | Companion firmware |
 | [`get/set bluetooth.mac`](cli_commands.md#view-or-change-the-bluetooth-address-ble-companion) | Every Companion build with Bluetooth |
 | [`get/set display.touch`](cli_commands.md#show-touchscreen-touch-areas) | Touchscreen Companions using the shared UI; text terminal and framed CLI; off after reboot |
+| [`get/set display.inbox`](cli_commands.md#set-companion-inbox-behavior) | Shared Companion UI; `history` (default), `pending`, or `unread`; saved across reboot |
 | [`uf2reset`](cli_commands.md#enter-the-uf2-bootloader-nrf52-only) | Every nRF52 Companion text terminal and local command `0x42`; local only |
 
 Deprecated binary aliases remain receive-only for older clients; new clients