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

TODO.md

@@ -21,3 +21,18 @@ list (`WorkoutLogListView`) has UI already waiting on these:
   machine settings, and its own "add exercise to a running workout" path
   (`Workouts Watch App/Views/WorkoutLogListView.swift`) doesn't snapshot
   `machineSettings` into the new log.
+
+## Wish list
+
+- **Pinned splits in the split picker**: add a "Pinned" section above "Recent" in
+  `SplitPickerSheet` (`Workouts/Views/WorkoutLogs/WorkoutLogsView.swift`), with a
+  swipe-to-pin action on the rows (and possibly on `SplitListView` for symmetry).
+  Storage decision (2026-07-08): a `pinnedSplitIDs` set in `UserDefaults` — matches
+  the existing preference precedent (`restSeconds`, `weightUnit`) and survives cache
+  rebuilds. Do **not** put an `isPinned` flag in `SplitDocument`: pinning a starter
+  seed would trigger the clone-on-edit fork, and `reconcileSeeds()` would overwrite
+  the flag with bundle bytes anyway. At read time, resolve stored IDs through
+  `sync.currentSplitID(for:)` (as `recentSplits` does) and prune IDs whose split no
+  longer exists; exclude pinned splits from the Recent section. Upgrade path if
+  cross-device pins ever matter: a tiny `Preferences/pins.json` iCloud document.
+  Watch, if wanted later: push the ID list via the `WCPayload` application context.