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.

OSGKeyboard/Services/FlowSessionManager.swift

@@ -111,6 +111,7 @@ final class FlowSessionManager: ObservableObject {
         }
 
         FlowSessionBridge.writeHeartbeat()
+        FlowSessionDarwin.postSessionChanged()
         isActive = true
         if let expires = FlowSessionBridge.sessionExpiresAt() {
             sessionExpiresAt = Date(timeIntervalSince1970: expires)
@@ -151,6 +152,7 @@ final class FlowSessionManager: ObservableObject {
 
         capture.stop()
         FlowSessionBridge.markSessionInactive()
+        FlowSessionDarwin.postSessionChanged()
         isActive = false
         sessionExpiresAt = nil
         sessionWarning = nil
@@ -186,6 +188,7 @@ final class FlowSessionManager: ObservableObject {
         }
 
         FlowSessionBridge.markSessionActive(duration: duration)
+        FlowSessionDarwin.postSessionChanged()
         isActive = true
         sessionExpiresAt = Date().addingTimeInterval(duration)