Derive isRecording from coordinator state machine

[Newarr]

Problem

isRecording is a writable stored property on AppCoordinator, set by ContentView's 100ms polling loop via synchronizeDerivedState(). Any consumer reading this property outside that loop -- such as a menu bar controller or termination guard -- sees stale values when the main window is hidden.

This is a prerequisite for the menu bar background mode feature, where recording state must be accurate without any visible window.

Solution

Replace the writable _isRecording / getter / setter with a computed property that derives directly from the coordinator's authoritative MeetingState:

Before	After
@ObservationIgnored nonisolated(unsafe) private var _isRecording = false	var isRecording: Bool { if case .recording = state { return true }; return false }
Set by ContentView polling loop	Derived from state machine
Stale when window hidden	Always accurate

Changed files

File	Change
AppCoordinator.swift	Replace stored property with computed property
ContentView.swift	Remove coordinator.isRecording = currentViewState.isRunning setter call

Test plan

• Start recording, verify isRecording returns true
• Stop recording, verify isRecording returns false
• Close main window while recording, verify isRecording still reflects true state
• Existing ui-smoke tests pass (no behavior change for consumers that only read the property)

PR 1 of 4 in the menu bar background mode series.
Next: #2 Extract service initialization for headless operation