Add reminder notifications to schedules

- ScheduleDocument/Schedule gain optional reminderMinutes (minutes from
  midnight; decode-compatible, no schema bump)
- New Workout form gets a Reminder section (toggle + time picker, hidden
  for Now; footer warns when notifications are denied)
- ReminderPlanner (pure, tested) derives notification triggers: daily
  repeating, per-weekday repeating for fixed days, one-shot for once
- ReminderScheduler resyncs pending requests from the cache on every
  change, multiplexed onto onCacheChanged after the watch push; asks
  notification permission only once a reminder actually exists

Shared/Model/Entities.swift

@@ -318,13 +318,15 @@ final class Schedule {
     var recurrenceRaw: String = ScheduleRecurrence.daily.rawValue
     var weekdays: [Int]?
     var date: Date?
+    /// Reminder time as minutes from midnight (local wall-clock); nil = no reminder.
+    var reminderMinutes: Int?
     var order: Int = 0
     var createdAt: Date = Date()
     var updatedAt: Date = Date()
     var jsonRelativePath: String = ""
 
     init(id: String, routineID: String, routineName: String, goalRaw: String?,
-         recurrenceRaw: String, weekdays: [Int]?, date: Date? = nil,
+         recurrenceRaw: String, weekdays: [Int]?, date: Date? = nil, reminderMinutes: Int? = nil,
          order: Int, createdAt: Date, updatedAt: Date, jsonRelativePath: String) {
         self.id = id
         self.routineID = routineID
@@ -333,6 +335,7 @@ final class Schedule {
         self.recurrenceRaw = recurrenceRaw
         self.weekdays = weekdays
         self.date = date
+        self.reminderMinutes = reminderMinutes
         self.order = order
         self.createdAt = createdAt
         self.updatedAt = updatedAt