feat: support iOS and iPadOS 17.4 - #104

Lowers the minimum from iOS 18.0 to 17.4. Normal can then be installed on the three iPads that stop at iPadOS 17 (iPad 6th gen, iPad Pro 10.5", iPad Pro 12.9" 2nd gen) and on devices that haven't updated past iOS 17.

Changes

• Deployment target is 17.4 for every target. 17.4 rather than 17.0 because Max Daily Limits uses DeviceActivityEvent(...includesPastActivity:), which needs 17.4. Every device on 17.0–17.3 can update to 17.7.
• MainTabView and SettingsView keep Tab on iOS 18 and later, line for line as on main. Tab needs iOS 18, so the else branch builds the same tabs with .tabItem and .tag for iOS 17.
• Control Center controls need iOS 18. The control types are marked @available(iOS 18.0, *), the widget bundle only registers them there, and reloadAllControls() is guarded.

Nothing changes on iOS 18 and later.

What iOS 17 doesn't get

• Control Center controls (Block All, Block Group).
• Unblock shortcuts with an NFC or QR key don't open Normal on their own. They show "Open Normal to unblock.", as on iOS 18.0 and 18.1 today. Location keys are unaffected.

Testing

• iOS 17.5, iPhone 15: 873 unit tests and all UI tests pass (37, with the orientation launch tests skipped). An earlier revision with the same iOS 17 tab code also passed on iPad (10th gen).
• iOS 27.0: the 14 UI tests that go through Settings pass.
• The app and widget binaries have a 17.4 minimum, and the iOS 18- and 26-only symbols they import are weak-linked.
• A SwiftData store holding every model, written on iOS 17.5, opens and re-saves on 18.6, 26.5 and 27.0.
• Not run on a real iPadOS 17 device. The simulator can't check blocking, schedules firing while the app is closed, daily limits, the website filter, hiding apps, the widget or the Live Activity, so this needs a TestFlight pass on one before release.