Widget freshness: best-effort background refresh of today's tile (BGAppRefreshTask)

[AtomicTrxn]

Split out of #116 (closed). The home-screen widget reads today's puzzle from the shared App Group container (CrosscueWidget.swift), which is only refreshed when the app runs. A user who hasn't opened Crosscue in a day or two sees a stale "today" tile.

Scope

• Register a BGAppRefreshTask (identifier e.g. dev.tomhess.crosscue.refresh) at launch; reschedule daily from the completion handler.
• On wake, run attemptIfNeeded() (downloads/imports today's puzzle) and refresh the widget timeline so the App Group snapshot is current.
• Document the limitation in the QA checklist: iOS controls cadence and may rarely fire it for low-engagement users — this is best-effort, not guaranteed.

Explicitly out of scope

• No push-based prefetch. The original iOS: improve Crosshare daily auto-download reliability (on-open + background) #116 Phase 2 "reliable FCM silent push" path is dead — friend boards (Design: private friend challenge boards via invite links #159) use Cloudflare Workers + D1 with no Firebase, and Design: private friend challenge boards via invite links #159's backend is result submission/ranking, not daily-puzzle fanout. Building APNs-direct daily push just for widget freshness isn't justified.
• No new entitlements beyond Background Modes (Background fetch/processing).

Why this is low priority

• iOS: improve Crosshare daily auto-download reliability (on-open + background) #116 Phase 1 already makes the in-app on-open experience fast and honest (retry + backoff, "fetching today's puzzle…" state).
• This issue only improves the widget glance for users who don't open the app — a polish item, not a correctness fix.

Acceptance

• BGAppRefreshTask registered at launch and rescheduled daily from completion
• On wake, today's puzzle is fetched (if missing) and the widget timeline reloaded
• QA checklist notes the best-effort / iOS-throttling caveat
• No regression to cold-start time or existing Phase 1 on-open behavior

Refs: #116 (origin), #159 (why no push)