Fix: Sleep score not shown on app relaunch after extended background

[Copilot]

HomeSummaryStore cached sleep scores in memory only. After iOS terminates the app (memory pressure, extended background), that cache is gone — the score shows "–" until a fresh API round-trip completes, and any network failure leaves it permanently missing.

Changes

Models/HomeSummary.swift

• Widened HomeSummary, SleepQualitySummary, FocusPredictionSummary, CircadianRhythmSummary, and ProcessingSummary from Decodable → Codable to enable encoding for UserDefaults storage.

Services/HomeSummaryStore.swift

• persistSummary(_:for:) / loadPersistedSummary(for:) — write/read JSON-encoded HomeSummary to UserDefaults keyed by date string (homeSummary.yyyy-MM-dd).
• cacheSummary() write-through — every path that updates the in-memory cache (direct fetch, month-cache, historical sync, polling) now also persists to UserDefaults.
• Warm-start in load() — seeds summary from UserDefaults before any network I/O, so the last-known score is visible instantly on relaunch:

// Before
summary = monthSummariesByDay[selectedMonthKey]?[selectedDayKey]

// After — falls back to durable cache when in-memory cache is cold
summary = monthSummariesByDay[selectedMonthKey]?[selectedDayKey]
       ?? loadPersistedSummary(for: selectedDayKey)

The month-cache path in load() now also calls cacheSummary() so bulk-fetched monthly summaries are persisted — previously they bypassed cacheSummary() entirely.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Bug: Sleep score missing</issue_title>
<issue_description>Describe the bug
Sleep score is not shown on launch when it's already computed.

To Reproduce
Steps to reproduce the behavior:

1. Wake up, view score after loading watch synced.
2. Score is shown.
3. Close app for 8 hours.
4. Reopen application.
5. Score is not shown.

Expected behavior
Score should be shown to user, even after closing the application for a prolonged period of time.

Additional context
Needs in-depth debugging to figure out.
Local storage should have the sleep score computed and stored. Need to figure out if it's even stored in local storage?
</issue_description>

<agent_instructions>Open pull request. Suggest best fixes.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Bug: Sleep score missing #4

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