Extract colors and apply cozy theme

[gsbernstein]

Summary

• Centralizes app theme colors, updates the palette to a warm "cozy" theme inspired by the app icon (including a true-black background/cards in dark mode), and makes theme a Settings option with two choices: System (the original default iOS colors) and Cozy (the warm palette, with a true-black dark mode).
• Colors are modeled as a ThemeColorPalette protocol with one conforming struct per theme (SystemThemeColors, CozyThemeColors), rather than a set of functions that switch on the theme. AppTheme.colors hands back the palette for the active case, and call sites read colors.accent etc., or resolve a KeyPath<any ThemeColorPalette, Color> when the choice itself depends on some other value (e.g. sleep stage colors, sleep-goal status colors).
• The cozy palette's colors live in the asset catalog (Assets.xcassets/Cozy*.colorset) instead of inline RGB values, so they can be tweaked visually in Xcode's color picker.

Changes

Color extraction & cozy theme

• New ThemeColorPalette protocol (Utils/ThemeColorPalette.swift) with named colors for accents, card icons, status indicators, sleep stages, backgrounds, and primary/secondary (so "grace"/fallback colors can also be expressed as a keypath).
• Two conforming structs:
  • SystemThemeColors — the original default iOS colors (blue/purple/red/green/orange/indigo/gray, secondarySystemBackground/systemBackground/tertiarySystemBackground), recovered from before the cozy palette existed. Intentionally mirrors the OS's own semantic colors rather than the asset catalog, since there's nothing there to make tunable.
  • CozyThemeColors — reads every color from a generated Color.cozy* extension backed by an asset-catalog colorset (CozyAccent, CozyBackground, CozySleepDeep, etc.), each with its own light/dark variant. Dark mode uses a true-black background with a near-black card color.
• All views updated to read colors through the active palette instead of .blue, .purple, .red, etc.

Theme as a setting

• New AppTheme enum (system, cozy) threaded through the view hierarchy via EnvironmentValues.appTheme, with a colors: any ThemeColorPalette computed property.
• Where a color choice depends on some other value (sleep stage, sleep-goal status), that logic returns a KeyPath<any ThemeColorPalette, Color> (or nil for a fallback) instead of switching on the theme itself — e.g. HKCategoryValueSleepAnalysis.paletteColor, Constants.sleepGoalColor(difference:), SleepBank.statusColor. The call site resolves the keypath against the active palette (colors[keyPath:]).
• UserPreferences gains a persisted theme property (stored as a raw string, defaulting to cozy, falling back to cozy on an unrecognized value so future app versions can add themes without breaking older data).
• SettingsView adds an "Appearance" section with a theme picker; it resolves preferences.theme.colors directly rather than through the appTheme environment key, since it's presented as a sheet/inspector.
• ContentView injects the selected theme into the environment and applies it to the background and the app-wide tint, so default-tinted system controls (buttons, toggles, nav bar) follow the theme too.

Test plan

• Build and run on iPhone simulator
• In Settings, switch between System / Cozy and confirm accents, card icons, sleep stage colors, and backgrounds update immediately
• Toggle dark mode under each theme and confirm: System uses default iOS backgrounds, Cozy uses true black with near-black cards
• Confirm the sleep balance waterfall chart and sleep insights card follow the selected theme
• Quit and relaunch the app to confirm the selected theme persists
• Open Assets.xcassets in Xcode and confirm the new Cozy* colorsets show up correctly and are editable/previewable