docs(theme): explain the Material and Mornye theme layer

Add lib/theme/README.md (UI-73): what Mornye is, which files form the theme layer, how DynamicColorWrapper builds each style, how Mornye maps onto Material 3 ColorScheme roles and AppTokens, and how widgets branch with context.isMornye and context.adaptiveIcon.

Derived from every file in lib/theme/, lib/models/theme_settings.dart, lib/providers/theme_provider.dart, lib/providers/runtime_profile_provider.dart, lib/widgets/mornye_chrome.dart and test/design_system_test.dart.

lib/theme/README.md

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+# Theme Layer
+
+The app has two visual styles, chosen in Appearance settings and stored as
+`AppThemeStyle { material, mornye }` (`lib/models/theme_settings.dart`,
+preference key `theme_style`, loaded by `lib/providers/theme_provider.dart`):
+
+- **Material**: Material 3 with dynamic color or a seed color, optional AMOLED
+  surfaces, and Material 3 Expressive motion and components.
+- **Mornye**: the Flutter counterpart of Mornye's iPhone visual system: iOS
+  system colors, Cupertino typography and icons, and glass surfaces. Its
+  palette is kept independent of wallpaper colors.
+
+Both styles still produce a Flutter `ThemeData` with `useMaterial3: true`, so
+widgets read `Theme.of(context).colorScheme` and `textTheme` the same way.
+Mornye maps its colors onto Material 3 `ColorScheme` roles instead of
+replacing them.
+
+## Files
+
+| File | Role |
+| --- | --- |
+| `lib/theme/dynamic_color_wrapper.dart` | `DynamicColorWrapper`, used by `lib/app.dart`. Builds the light and dark `ThemeData` for the selected style |
+| `lib/theme/app_theme.dart` | `AppTheme.light` and `AppTheme.dark`: Material 3 component themes over a `ColorScheme` |
+| `lib/theme/app_tokens.dart` | `AppTokens`, a `ThemeExtension` with the shared radii, spacing, cover sizes and header metrics |
+| `lib/theme/mornye_theme.dart` | `MornyeTheme`: the Mornye `ThemeExtension`, its `ThemeData` builder, `context.isMornye`, `MornyeSettingsTheme` |
+| `lib/theme/mornye_icons.dart` | `mornyeIconFor` (Material icon to Cupertino icon) and `context.adaptiveIcon` |
+| `lib/theme/material_symbols.dart` | Material Symbols Rounded counterparts of `Icons.*` for the Material style |
+| `lib/theme/material_expressive.dart` | `materialExpressiveEnabled` and `MaterialExpressiveScope`, the bridge to the Material UI and Material 3 Expressive packages |
+| `lib/theme/app_motion.dart` | `AppMotionSpec`, `SpringCurve` and `AppMotion` spring presets |
+| `lib/theme/cover_palette.dart` | `CoverPalette`, `HeaderPalette` and `CoverPaletteBuilder`: colors extracted from artwork |
+
+## How a Theme Is Built
+
+`DynamicColorWrapper` watches `themeProvider` and the custom font provider.
+
+- Material: `DynamicColorBuilder` supplies the wallpaper scheme when dynamic
+  color is on; otherwise `ColorScheme.fromSeed` uses the seed color. AMOLED
+  replaces the dark surfaces with black and near-black. `AppTheme` then adds
+  component themes and registers `AppTokens.standard`.
+- Mornye: `MornyeTheme.build(brightness, ...)` takes the Mornye accent
+  (`MornyeAccent`, red by default), the system-font and custom-font choices,
+  glass clarity (default 0.75) and glass outlines. It seeds a `ColorScheme`
+  from the accent and overrides the roles with iOS grouped colors (for
+  example `#F2F2F7` and `#1C1C1E`). Text uses Cupertino styles. The font is
+  the custom font when one is set; otherwise the Cupertino system font
+  (`CupertinoSystemText`, SF on Apple) on iOS and macOS or when the system
+  font is chosen, and Inter elsewhere. It registers its own
+  `MornyeTheme.tokens`, a copy of `AppTokens.standard` with different radii
+  (smaller for most shapes, larger for sheets) and iOS title sizes, and
+  removes ink splashes.
+
+`MornyeTheme.build` caches up to 16 base themes; changing only glass clarity
+or outlines reuses the cached theme. Use `MornyeTheme.fromContext` to rebuild
+a local surface (another brightness or chrome color) without losing the
+user's Mornye preferences.
+
+## Using the Layer in Widgets
+
+- Read tokens with `context.tokens`, not `Theme.of(context).extension`. It
+  falls back to `AppTokens.standard` outside a themed subtree.
+- Branch on `context.isMornye`, which is true when the nearest `Theme` carries
+  a `MornyeTheme` extension. Because it follows the nearest `Theme`, a
+  subtree can be switched by wrapping it in another theme.
+- Keep `Icons.*` at call sites and pass them through `context.adaptiveIcon`;
+  Mornye then shows the Cupertino glyph from `mornyeIconFor`.
+- Gate Material 3 Expressive motion with `materialExpressiveEnabled` (false
+  for Mornye and when the OS asks for reduced motion). `AppMotion` presets
+  return the call site's fallback spec in those cases.
+- Wrap settings pages in `MornyeSettingsTheme` for iOS inset grouped
+  surfaces; it does nothing in the Material style.
+
+## Mornye Outside This Folder
+
+- Glass rendering level: `MornyeGlassLevel` (`flat`, `frosted`, `liquid`) and
+  `mornyeGlassLevelProvider` in `lib/providers/runtime_profile_provider.dart`
+  choose how much glass a device renders.
+- Mornye widgets live in `lib/widgets/mornye_*.dart`, for example `MornyeGlass`
+  in `lib/widgets/mornye_chrome.dart`. Mornye-only screens are
+  `lib/screens/mornye_library_screen.dart` and
+  `lib/screens/track_metadata_mornye.dart`; most other screens branch on
+  `context.isMornye`.
+
+## Tests
+
+`test/design_system_test.dart` checks that `AppTokens` is registered on the
+Material themes (`AppTokens.standard`) and on Mornye (`MornyeTheme.tokens`),
+the Mornye dark accent, and Mornye's font families and letter spacing. Mornye
+widgets have their own `test/mornye_*_test.dart` suites.