Extract colors and apply cozy theme

[gsbernstein]

Summary

• Commit 1: Centralizes all app theme colors in AppColors.swift and updates views to reference named colors instead of inline system colors. No visual changes.
• Commit 2: Updates the palette to a warm, cozy theme inspired by the new app icon — coral accents, cream backgrounds, dusty mauve/brown sleep-stage colors, and sage/terracotta status tones.

Changes

Color extraction (AppColors.swift)

• New AppColors enum in Utils/AppColors.swift with named colors for accents, card icons, status indicators, and sleep stages
• All views updated to use AppColors.* instead of .blue, .purple, .red, etc.

Cozy theme

• Accent / card icons: Warm coral (#f8987c) from the app icon
• Recent sleep / REM: Dusty mauve brown (#9b786d)
• Status positive/negative: Muted sage green and warm terracotta
• Sleep stages: Warm browns, plum, and peach tones
• Asset catalog: AccentColor, BackgroundBehindCards, and CardBackground updated with adaptive cream/brown light and dark values
• All AppColors use adaptive light/dark variants via UIColor dynamic provider

Test plan

• Build and run on iPhone simulator
• Verify card icons and accents use warm coral instead of blue/purple
• Check sleep stage colors in expanded recent-sleep rows
• Toggle dark mode and confirm backgrounds and accents adapt correctly
• Confirm Settings sliders use the new accent color