Extract application orchestration from MainWindow

[singular0]

MainWindow owns connection switching, device identity, channel caching, SQLite history, pending
sends, removals, unread state and presentation. Extract a DeviceSession or ApplicationController
so the UI only binds state and invokes commands.

Reference: src/ui/main_window.h:86 and src/ui/main_window.cpp.

The controller should own:

• CompanionClient and connection/session state.
• Active-device identity and slot-to-channel identity resolution.
• Message ingestion and persistence.
• Pending sends and channel mutations.
• Channel cache and persistent selection.
• Application-facing error and status reporting.

Worth doing before direct messages and contacts add a second and third conversation kind to the same
class.

---

Worth landing before #1 and #2 add more conversation kinds to MainWindow.