Add Catppuccin Latte, Dracula, Alucard, Nord and Tokyo Night themes

Add five new selectable themes alongside the existing Catppuccin Mocha
and Gruvbox Dark, each built through the shared buildAppTheme so they
stay visually consistent. Palettes are taken from each project's
official source; a few intermediate surface shades are interpolated
where the canonical palette omits them (noted in the theme files).

Register the new themes in main.dart and add their dropdown entries in
settings.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/lib/theme/catppuccin_latte_theme.dart

@@ -0,0 +1,92 @@
+import 'package:flutter/material.dart';
+import 'app_colors.dart';
+import 'theme_builder.dart';
+
+abstract final class CatppuccinLatteColors {
+  static const Color crust = Color(0xFFdce0e8);
+  static const Color mantle = Color(0xFFe6e9ef);
+  static const Color base = Color(0xFFeff1f5);
+  static const Color surface0 = Color(0xFFccd0da);
+  static const Color surface1 = Color(0xFFbcc0cc);
+  static const Color surface2 = Color(0xFFacb0be);
+  static const Color overlay0 = Color(0xFF9ca0b0);
+  static const Color overlay1 = Color(0xFF8c8fa1);
+  static const Color overlay2 = Color(0xFF7c7f93);
+  static const Color subtext0 = Color(0xFF6c6f85);
+  static const Color subtext1 = Color(0xFF5c5f77);
+  static const Color text = Color(0xFF4c4f69);
+
+  static const Color rosewater = Color(0xFFdc8a78);
+  static const Color flamingo = Color(0xFFdd7878);
+  static const Color pink = Color(0xFFea76cb);
+  static const Color mauve = Color(0xFF8839ef);
+  static const Color red = Color(0xFFd20f39);
+  static const Color maroon = Color(0xFFe64553);
+  static const Color peach = Color(0xFFfe640b);
+  static const Color yellow = Color(0xFFdf8e1d);
+  static const Color green = Color(0xFF40a02b);
+  static const Color teal = Color(0xFF179299);
+  static const Color sky = Color(0xFF04a5e5);
+  static const Color sapphire = Color(0xFF209fb5);
+  static const Color blue = Color(0xFF1e66f5);
+  static const Color lavender = Color(0xFF7287fd);
+}
+
+const ColorScheme _catppuccinLatteColorScheme = ColorScheme(
+  brightness: Brightness.light,
+
+  // Surfaces — AppBar (darkest) → NavRail → body (lightest)
+  surface: CatppuccinLatteColors.base,
+  surfaceContainerLowest: CatppuccinLatteColors.crust,
+  surfaceContainerLow: CatppuccinLatteColors.mantle,
+  surfaceContainer: CatppuccinLatteColors.surface0,
+  surfaceContainerHigh: CatppuccinLatteColors.surface1,
+  surfaceContainerHighest: CatppuccinLatteColors.surface2,
+  onSurface: CatppuccinLatteColors.text,
+  onSurfaceVariant: CatppuccinLatteColors.subtext1,
+  outline: CatppuccinLatteColors.overlay1,
+  outlineVariant: CatppuccinLatteColors.overlay0,
+
+  // Primary — Save button, selected nav indicators
+  primary: CatppuccinLatteColors.mauve,
+  onPrimary: CatppuccinLatteColors.base,
+  primaryContainer: CatppuccinLatteColors.surface0,
+  onPrimaryContainer: CatppuccinLatteColors.text,
+
+  // Secondary — Test button (not-passing state)
+  secondary: CatppuccinLatteColors.blue,
+  onSecondary: CatppuccinLatteColors.base,
+  secondaryContainer: CatppuccinLatteColors.surface1,
+  onSecondaryContainer: CatppuccinLatteColors.text,
+
+  // Tertiary — swipe-to-unread background
+  tertiary: CatppuccinLatteColors.teal,
+  onTertiary: CatppuccinLatteColors.base,
+  tertiaryContainer: CatppuccinLatteColors.surface0,
+  onTertiaryContainer: CatppuccinLatteColors.text,
+
+  // Error
+  error: CatppuccinLatteColors.red,
+  onError: CatppuccinLatteColors.base,
+  errorContainer: CatppuccinLatteColors.maroon,
+  onErrorContainer: CatppuccinLatteColors.base,
+
+  // Inverse / scrim
+  inverseSurface: CatppuccinLatteColors.text,
+  onInverseSurface: CatppuccinLatteColors.base,
+  inversePrimary: CatppuccinLatteColors.mauve,
+  scrim: CatppuccinLatteColors.crust,
+  shadow: CatppuccinLatteColors.text,
+);
+
+final ThemeData catppuccinLatteLightTheme = buildAppTheme(
+  colorScheme: _catppuccinLatteColorScheme,
+  appColors: const AppColorExtension(
+    success: CatppuccinLatteColors.green,
+    onSuccess: CatppuccinLatteColors.base,
+    warning: CatppuccinLatteColors.peach,
+    onWarning: CatppuccinLatteColors.base,
+    info: CatppuccinLatteColors.blue,
+    onInfo: CatppuccinLatteColors.base,
+  ),
+);