Run mode activation and deactivation wired through `AppWindowMode`

[gwillish]

Per redesign-plan.md M6c. Depends on #116.

Critical file: Encounter/ContentView.swift (macOS/iPadOS branch)

• Run button in EncounterEditorView toolbar calls windowMode = .runningEncounter(session) via a callback or binding passed through the view hierarchy (per ADR-0041 explicit DI)
• Pause: calls session.pause() → SessionStore.save() → resets windowMode = .encounterPrep
• End: calls store.clearSession(for:) → resets windowMode = .encounterPrep

Test first: Run button sets correct windowMode; Pause resets to .encounterPrep with session saved; End clears session.