feat: clone Opal's rules feature with hard block enforcement

Rebuild the app around recurring screen-time blocking rules modeled on
Opal's My Apps tab:

- Onboarding with FamilyControls Screen Time authorization
- Apps home with Blocked Apps tiles and live rule cards
- New Rule sheet: Schedule/Time Limit/Open Limit types + preset gallery
- Rule editors: time windows (incl. overnight), day picker, app selection
  via FamilyActivityPicker (Block/Allow Only), rename, Hold to Commit
- Hard Mode: active hard rules cannot be edited, disabled, deleted, or
  unblocked until their window ends; soft rules pause until next window
- Shield enforcement through per-rule ManagedSettingsStore + RuleEnforcer
- 73 unit tests (Swift Testing) + 16 UI tests (XCUITest) with launch-arg
  harness for in-memory storage, mocked authorization, seeded scenarios
- docs/RULES_FEATURE_SPEC.md: spec derived from the reference recording

Known gap: background window transitions and time/open-limit thresholds
need a DeviceActivityMonitor extension; shields currently sync while the
app is running.

SeveredUITests/UITestSupport.swift

@@ -0,0 +1,48 @@
+//
+//  UITestSupport.swift
+//  SeveredUITests
+//
+
+import XCTest
+
+extension XCUIApplication {
+    /// Launches the app in UI-testing mode: in-memory storage, mocked Screen
+    /// Time authorization, and no shield side effects.
+    static func launchSevered(
+        onboardingCompleted: Bool = true,
+        seedScenario: String? = nil
+    ) -> XCUIApplication {
+        let app = XCUIApplication()
+        var arguments = ["-ui-testing"]
+        arguments.append(onboardingCompleted ? "-onboarding-completed" : "-onboarding-required")
+        if let seedScenario {
+            arguments.append("-seed-scenario=\(seedScenario)")
+        }
+        app.launchArguments = arguments
+        app.launch()
+        return app
+    }
+}
+
+extension XCUIApplication {
+    /// Finds an element by accessibility identifier regardless of how SwiftUI
+    /// exposes it (Other, StaticText, Button, …).
+    func element(_ identifier: String) -> XCUIElement {
+        descendants(matching: .any).matching(identifier: identifier).firstMatch
+    }
+}
+
+extension XCUIElement {
+    /// Asserts the element appears within the timeout, then returns it.
+    @discardableResult
+    func waitToAppear(
+        timeout: TimeInterval = 5, file: StaticString = #filePath, line: UInt = #line
+    ) -> XCUIElement {
+        XCTAssertTrue(
+            waitForExistence(timeout: timeout),
+            "Expected \(self) to exist within \(timeout)s",
+            file: file, line: line
+        )
+        return self
+    }
+}