iOS 1.0 RC 1: battery and heat work, version label, web player redraw tweaks

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ios/Stemify/Core/AppActivity.swift

@@ -0,0 +1,38 @@
+import Foundation
+
+/// Whether anyone is looking at the app: the phone UI in the foreground, or CarPlay connected.
+/// Work that only feeds a screen (stemd polling, a shallow output buffer for snappy faders)
+/// checks this, so a locked phone playing music does as little as it can.
+enum AppActivity {
+    private static let lock = UnfairLock()
+    nonisolated(unsafe) private static var phone = true
+    nonisolated(unsafe) private static var car = false
+    nonisolated(unsafe) private static var waiters: [CheckedContinuation<Void, Never>] = []
+
+    static var isActive: Bool { lock.with { phone || car } }
+
+    static func setPhone(_ on: Bool) { update { phone = on } }
+    static func setCar(_ on: Bool) { update { car = on } }
+
+    private static func update(_ change: () -> Void) {
+        let ready = lock.with { () -> [CheckedContinuation<Void, Never>] in
+            change()
+            guard phone || car else { return [] }
+            defer { waiters = [] }
+            return waiters
+        }
+        ready.forEach { $0.resume() }
+    }
+
+    /// Returns once the app is active, at once if it already is.
+    static func untilActive() async {
+        await withCheckedContinuation { (c: CheckedContinuation<Void, Never>) in
+            let now = lock.with { () -> Bool in
+                if phone || car { return true }
+                waiters.append(c)
+                return false
+            }
+            if now { c.resume() }
+        }
+    }
+}