Sleep hypnogram alignment + press-and-hold readout, sync spinner fixes

[saksham2001]

TestFlight feedback fixes (build 4 report, 06-sleep-chart-misaligned):

• Hypnogram labels aligned to bars — labels and Canvas bars now share one laneFraction/plotInsets source of truth (Charts.swift)
• Press-and-hold stage readout — hold + drag shows e.g. DEEP · 3:05 – 3:51 AM with indicator line + haptics; quick swipes still page the carousel
• "Loading spinner lags" — SyncProgressBar rewritten as wall-clock TimelineView; isSyncing now transition-only (was invalidating the whole tab tree per sync packet); ConnectionStatusPill pulse lifecycle fixed; per-render SwiftData fetch removed from Today body
• Empty sleep day — "wear your ring" explainer now only in the architecture card; hero/stage cards show dashes

Tests: 906 pass, incl. 16 new in SleepHypnogramMathTests. Verified in simulator (alignment, scrubber, paging, empty state). Merged with latest main.