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.

Severed/Models/Weekday.swift

@@ -0,0 +1,65 @@
+//
+//  Weekday.swift
+//  Severed
+//
+
+import Foundation
+
+/// A day of the week, using `Calendar` weekday numbering (1 = Sunday … 7 = Saturday).
+enum Weekday: Int, CaseIterable, Codable, Hashable, Sendable {
+    case sunday = 1
+    case monday = 2
+    case tuesday = 3
+    case wednesday = 4
+    case thursday = 5
+    case friday = 6
+    case saturday = 7
+
+    static let weekdays: Set<Weekday> = [.monday, .tuesday, .wednesday, .thursday, .friday]
+    static let weekends: Set<Weekday> = [.saturday, .sunday]
+    static let everyDay: Set<Weekday> = Set(Weekday.allCases)
+
+    /// Display order used by the day picker, matching the reference UI: S M T W T F S.
+    static let displayOrder: [Weekday] = [
+        .sunday, .monday, .tuesday, .wednesday, .thursday, .friday, .saturday,
+    ]
+
+    /// Single-letter label for the circular day toggles.
+    var shortLabel: String {
+        switch self {
+        case .sunday, .saturday: "S"
+        case .monday: "M"
+        case .tuesday, .thursday: "T"
+        case .wednesday: "W"
+        case .friday: "F"
+        }
+    }
+
+    /// Three-letter abbreviation used in custom day summaries ("Mon, Wed, Fri").
+    var abbreviation: String {
+        switch self {
+        case .sunday: "Sun"
+        case .monday: "Mon"
+        case .tuesday: "Tue"
+        case .wednesday: "Wed"
+        case .thursday: "Thu"
+        case .friday: "Fri"
+        case .saturday: "Sat"
+        }
+    }
+}
+
+extension Set<Weekday> {
+    /// Human-readable summary shown next to the day picker and in rule details:
+    /// "Weekdays", "Weekends", "Every day", "Never", or a list like "Mon, Wed, Fri".
+    var summary: String {
+        if self == Weekday.everyDay { return "Every day" }
+        if self == Weekday.weekdays { return "Weekdays" }
+        if self == Weekday.weekends { return "Weekends" }
+        if isEmpty { return "Never" }
+        return Weekday.displayOrder
+            .filter(contains)
+            .map(\.abbreviation)
+            .joined(separator: ", ")
+    }
+}