Queue document writes durably and surface sync trouble in the UI

iCloud Drive writes now flow through a persistent WriteBacklog sidecar
(drained with backoff, flushed on backgrounding, wiped with the cache on
account change), so a save can never be lost to a transient coordinator
error. A status banner on the workout list surfaces stuck syncing.
Also: the split picker gains a Recent section with day labels, split
rows fold SplitItem into SplitListView, and list rows dim the multiply
sign in sets-by-reps.

Claude-Session: https://claude.ai/code/session_01HJDQQDA9QdP8zByg43H5v3

Shared/Utils/Date+Extensions.swift

@@ -39,6 +39,21 @@ extension Date {
     var abbreviatedWeekday: String { Self.weekdayAbbrev.string(from: self) }
     var dayOfMonth: Int { Calendar.current.component(.day, from: self) }
 
+    /// "Today", "Yesterday", or "N days ago" by calendar-day difference.
+    func daysAgoLabel(relativeTo now: Date = Date()) -> String {
+        let calendar = Calendar.current
+        let days = calendar.dateComponents(
+            [.day],
+            from: calendar.startOfDay(for: self),
+            to: calendar.startOfDay(for: now)
+        ).day ?? 0
+        switch days {
+        case ..<1: return "Today"
+        case 1: return "Yesterday"
+        default: return "\(days) days ago"
+        }
+    }
+
     func humanTimeInterval(to other: Date) -> String {
         let interval = other.timeIntervalSince(self)
         let hours = Int(interval) / 3600