feat: complete Phase 4 batch F, keyboard i18n, and Pages app icon

Remove KeyboardL10n hard-coded fallbacks in favor of ExtL10n and extension
Localizable.strings. Add Flow session expiry hints, Darwin cross-process
notifications, session monitor on the keyboard, and app icon on GitHub Pages.

OSGKeyboardShared/Services/FlowSessionDarwin.swift

@@ -0,0 +1,62 @@
+// FlowSessionDarwin.swift
+// OSGKeyboard · Shared
+//
+// Cross-process Darwin notification when the host app changes Flow session
+// state (start, extend, end). Keyboard extension listens without polling alone.
+
+import Foundation
+
+public enum FlowSessionDarwin {
+    public static let notificationName = "com.osgkeyboard.flow.session.changed"
+
+    public static func postSessionChanged() {
+        CFNotificationCenterPostNotification(
+            CFNotificationCenterGetDarwinNotifyCenter(),
+            CFNotificationName(notificationName as CFString),
+            nil,
+            nil,
+            true
+        )
+    }
+}
+
+/// Observes Flow session Darwin notifications on a background thread; invokes
+/// `handler` on the main actor.
+public final class FlowSessionDarwinObserver {
+    private final class Box: @unchecked Sendable {
+        let handler: @MainActor () -> Void
+        init(handler: @escaping @MainActor () -> Void) { self.handler = handler }
+    }
+
+    private let box: Box
+    private let token: UnsafeMutableRawPointer
+
+    public init(handler: @escaping @MainActor () -> Void) {
+        let box = Box(handler: handler)
+        self.box = box
+        self.token = Unmanaged.passRetained(box).toOpaque()
+
+        CFNotificationCenterAddObserver(
+            CFNotificationCenterGetDarwinNotifyCenter(),
+            token,
+            { _, observer, _, _, _ in
+                guard let observer else { return }
+                let box = Unmanaged<Box>.fromOpaque(observer).takeUnretainedValue()
+                Task { @MainActor in box.handler() }
+            },
+            FlowSessionDarwin.notificationName as CFString,
+            nil,
+            .deliverImmediately
+        )
+    }
+
+    deinit {
+        CFNotificationCenterRemoveObserver(
+            CFNotificationCenterGetDarwinNotifyCenter(),
+            token,
+            CFNotificationName(FlowSessionDarwin.notificationName as CFString),
+            nil
+        )
+        Unmanaged<Box>.fromOpaque(token).release()
+    }
+}

OSGKeyboardShared/Services/KeyboardState.swift

@@ -69,6 +69,8 @@ public final class KeyboardState: ObservableObject {
     @Published public var onDeviceSupported: Bool = false
     /// Seconds remaining in the current utterance (Flow tap-to-talk).
     @Published public var utteranceRemainingSeconds: Int = Int(FlowSessionKeys.maxUtteranceDuration)
+    /// Whether the host app's Flow voice session is currently valid.
+    @Published public var flowSessionActive: Bool = false
     /// "local" → ASR only, no LLM. "cloud" → ASR + optional LLM polish.
     @Published public var engineMode: String = "cloud"