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/Persistence/WorkoutsModelContainer.swift

@@ -18,6 +18,14 @@ enum WorkoutsModelContainer {
         URL.applicationSupportDirectory.appending(path: "Workouts.store")
     }
 
+    /// Sidecar file holding queued-but-unwritten document writes (`WriteBacklog`,
+    /// iOS only). Lives beside the cache store — outside the iCloud container —
+    /// and is wiped with it on an account change: a backlog from one account must
+    /// never drain into another account's container.
+    static var pendingWritesURL: URL {
+        URL.applicationSupportDirectory.appending(path: "PendingWrites.json")
+    }
+
     static func make() -> ModelContainer {
         let schema = Schema([Split.self, Exercise.self, Workout.self, WorkoutLog.self])
         ensureStoreDirectoryExists()
@@ -70,6 +78,7 @@ enum WorkoutsModelContainer {
         let stored = UserDefaults.standard.data(forKey: identityTokenKey)
         guard current != stored else { return }
         wipeStore()
+        try? FileManager.default.removeItem(at: pendingWritesURL)
         UserDefaults.standard.set(current, forKey: identityTokenKey)
     }