feat(flow): implement ABCD session policy and host return whitelist

- Scheme A: on-demand session start, inactivity-based expiry, handoff auto-recording
- Scheme B: cold-start overlay with swipe guidance and return alert
- Scheme C+D: HostAppURLRegistry (20 apps) and sourceApplication capture
- Settings: skip app switch toggle and inactivity duration picker
- Add LSApplicationQueriesSchemes for canOpenURL checks

Co-authored-by: Rocky <hkgood@users.noreply.github.com>

OSGKeyboardShared/Models/FlowInactivityDuration.swift

@@ -0,0 +1,46 @@
+// FlowInactivityDuration.swift
+// OSGKeyboard · Shared
+//
+// User-selectable Flow session inactivity timeout. The timer resets after
+// each completed utterance (and on session start).
+
+import Foundation
+
+public enum FlowInactivityDuration: String, CaseIterable, Identifiable, Sendable, Codable {
+    case tenMinutes = "10m"
+    case thirtyMinutes = "30m"
+    case threeHours = "3h"
+    case twelveHours = "12h"
+    case twentyFourHours = "24h"
+
+    public var id: String { rawValue }
+
+    public static let `default`: FlowInactivityDuration = .twelveHours
+
+    public var timeInterval: TimeInterval {
+        switch self {
+        case .tenMinutes: return 10 * 60
+        case .thirtyMinutes: return 30 * 60
+        case .threeHours: return 3 * 60 * 60
+        case .twelveHours: return 12 * 60 * 60
+        case .twentyFourHours: return 24 * 60 * 60
+        }
+    }
+
+    public var labelKey: String {
+        switch self {
+        case .tenMinutes: return "settings.flow.inactivity.10m"
+        case .thirtyMinutes: return "settings.flow.inactivity.30m"
+        case .threeHours: return "settings.flow.inactivity.3h"
+        case .twelveHours: return "settings.flow.inactivity.12h"
+        case .twentyFourHours: return "settings.flow.inactivity.24h"
+        }
+    }
+
+    public static func fromStored(_ raw: String?) -> FlowInactivityDuration {
+        guard let raw, let value = FlowInactivityDuration(rawValue: raw) else {
+            return .default
+        }
+        return value
+    }
+}