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

.github/workflows/run-unit-tests.yml

@@ -109,6 +109,7 @@ jobs:
       - name: Verify Companion Bluetooth settings and WebConfig
         run: |
           python3 -B test/test_display_power.py
+          python3 -B test/test_companion_inbox.py
           python3 -B test/test_heltec_r8_external_power.py
           python3 -B test/test_bluetooth_mac_contract.py
           python3 -B test/test_companion_settings_persistence_contract.py