Fix launch crash, infinite spinner, and HealthKit data loading

[gsbernstein]

Closed as superseded. Two of the three fixes here are now on master: the SwiftData store-reset recovery, and awaiting HealthKit queries (done properly in #40 via HKSampleQueryDescriptor). The remaining diagnostics and Grant Access work moved to #30.

One item was not carried forward: ContentView still inserts UserPreferences into the model context from inside a computed property read during body evaluation. Master's ContentView has since been restructured substantially, so reapplying that refactor belongs in its own PR against current master rather than in this stale branch.

Problem

TestFlight crash at launch (BedtimeApp.swift:23), plus an infinite loading spinner after reinstall, plus sleep data not appearing.

Root causes

1. SwiftData store incompatible after Replace max sleep hours with earliest reasonable bedtime #20 — ModelContainer creation failed and hit fatalError before any UI loaded.
2. Cancelled .task left HealthKit in .loading — seeding preferences and fetching HealthKit data shared one .task on a view whose identity changed when @Query populated, so SwiftUI cancelled it mid-flight and the permission state never advanced.
3. HealthKit queries were fire-and-forget — loadSleepData() returned before HKSampleQuery callbacks ran, so the UI could leave the loading state before data arrived.