Clarify package and type boundaries

[singular0]

The model directory combines domain values, SQLite persistence and Qt presentation models.
CompanionClient imports those model types, and Channel represents both a live secret-bearing
channel and a secret-free cached one.

Target layout:

domain/       DeviceId, ChannelId, ChannelSecret, Message
protocol/     wire DTOs, codecs, client, transports
persistence/  HistoryStore, SettingsStore, migrations
app/          DeviceSession and application workflows
ui/models/    QAbstractListModel adapters
ui/           widgets and delegates

Tasks:

• Introduce validated DeviceId, ChannelId, ChannelSecret and ChannelSlot value types
  instead of interchangeable QByteArray and int parameters.
• Split the live channel representation from its cached summary.
• Move History into persistence and the Qt list models under presentation.
• Keep wire DTO parsing in the protocol layer and map it into domain types at the application
  boundary.