Fix iOS widget background refresh

[austin-smith]

What Changed

• Enable background fetch for iOS widget refreshes.
• Use the iOS 27 task submission API while retaining iOS 26 support, and build iOS CI with Xcode 27.
• Remove redundant request cancellation, log registration and submission results, and avoid retaining completed refresh tasks through the expiration handler.
• Replace widget snapshots atomically, add cancellation regression tests, and update the widget documentation.

Why

The app registered and scheduled background refresh tasks without declaring the required fetch background mode, preventing iOS from running them. Atomic snapshot replacement also prevents widgets from encountering a missing file during an update.

Validation

• 225 macOS tests passed, including active and queued refresh cancellation.
• Signed macOS Debug and unsigned iOS Debug/Release builds passed with Xcode 27.
• Checked the built iOS background-mode and task-identifier metadata; snapshot filesystem smoke checks passed.
• SwiftLint and git diff --check passed.
• Physical-device background launch, expiration, and locked-device behavior have not been tested.

[chatgpt-codex-connector]

Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-19T02:37:59.440967Z	190b07a	Draft marked ready

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[chatgpt-codex-connector]

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Reviewed commit: 190b07af0e

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