Add per-user notification time preferences

Allow users to customize when they receive notification reminders:
- Add hour fields to NotificationPreference model
- Add timezone conversion utilities (localHourToUtc, utcHourToLocal)
- Add time picker UI for iOS (wheel picker in sheet)
- Add time picker UI for Android (hour chip selector dialog)
- Times stored in UTC, displayed in user's local timezone
- Add localized strings for time picker UI

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

iosApp/iosApp/Helpers/DateUtils.swift

@@ -176,4 +176,73 @@ enum DateUtils {
         let today = Calendar.current.startOfDay(for: Date())
         return date < today
     }
+
+    // MARK: - Timezone Conversion Utilities
+
+    /// Convert a local hour (0-23) to UTC hour
+    /// - Parameter localHour: Hour in the device's local timezone (0-23)
+    /// - Returns: Hour in UTC (0-23)
+    static func localHourToUtc(_ localHour: Int) -> Int {
+        let now = Date()
+        let calendar = Calendar.current
+
+        // Create a date with the given local hour
+        var components = calendar.dateComponents(in: TimeZone.current, from: now)
+        components.hour = localHour
+        components.minute = 0
+        components.second = 0
+
+        guard let localDate = calendar.date(from: components) else {
+            return localHour
+        }
+
+        // Get the hour in UTC
+        var utcCalendar = Calendar.current
+        utcCalendar.timeZone = TimeZone(identifier: "UTC")!
+        let utcHour = utcCalendar.component(.hour, from: localDate)
+
+        return utcHour
+    }
+
+    /// Convert a UTC hour (0-23) to local hour
+    /// - Parameter utcHour: Hour in UTC (0-23)
+    /// - Returns: Hour in the device's local timezone (0-23)
+    static func utcHourToLocal(_ utcHour: Int) -> Int {
+        let now = Date()
+
+        // Create a calendar in UTC
+        var utcCalendar = Calendar.current
+        utcCalendar.timeZone = TimeZone(identifier: "UTC")!
+
+        // Create a date with the given UTC hour
+        var components = utcCalendar.dateComponents([.year, .month, .day, .hour, .minute, .second], from: now)
+        components.hour = utcHour
+        components.minute = 0
+        components.second = 0
+
+        guard let utcDate = utcCalendar.date(from: components) else {
+            return utcHour
+        }
+
+        // Get the hour in local timezone
+        let localHour = Calendar.current.component(.hour, from: utcDate)
+
+        return localHour
+    }
+
+    /// Format an hour (0-23) to a human-readable 12-hour format
+    /// - Parameter hour: Hour in 24-hour format (0-23)
+    /// - Returns: Formatted string like "8:00 AM" or "2:00 PM"
+    static func formatHour(_ hour: Int) -> String {
+        switch hour {
+        case 0:
+            return "12:00 AM"
+        case 1..<12:
+            return "\(hour):00 AM"
+        case 12:
+            return "12:00 PM"
+        default:
+            return "\(hour - 12):00 PM"
+        }
+    }
 }