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/tokyo_night_theme.dart

@@ -0,0 +1,88 @@
+import 'package:flutter/material.dart';
+import 'app_colors.dart';
+import 'theme_builder.dart';
+
+/// Tokyo Night (the "Night" variant).
+abstract final class TokyoNightColors {
+  static const Color bgDarker = Color(0xFF0c0e14);
+  static const Color bgDark = Color(0xFF16161e);
+  static const Color bg = Color(0xFF1a1b26);
+  static const Color bgSoft = Color(0xFF1b1e2d);
+  static const Color bgElevated = Color(0xFF24283b);
+  static const Color bgHighlight = Color(0xFF292e42);
+  static const Color fgGutter = Color(0xFF3b4261);
+  static const Color terminalBlack = Color(0xFF414868);
+  static const Color comment = Color(0xFF565f89);
+  static const Color fgDark = Color(0xFFa9b1d6);
+  static const Color fg = Color(0xFFc0caf5);
+
+  static const Color blue = Color(0xFF7aa2f7);
+  static const Color cyan = Color(0xFF7dcfff);
+  static const Color teal = Color(0xFF73daca);
+  static const Color green = Color(0xFF9ece6a);
+  static const Color magenta = Color(0xFFbb9af7);
+  static const Color purple = Color(0xFF9d7cd8);
+  static const Color orange = Color(0xFFff9e64);
+  static const Color yellow = Color(0xFFe0af68);
+  static const Color red = Color(0xFFf7768e);
+  static const Color brightRed = Color(0xFFdb4b4b);
+}
+
+const ColorScheme _tokyoNightColorScheme = ColorScheme(
+  brightness: Brightness.dark,
+
+  // Surfaces — AppBar (darkest) → NavRail → body (lightest)
+  surface: TokyoNightColors.bg,
+  surfaceContainerLowest: TokyoNightColors.bgDark,
+  surfaceContainerLow: TokyoNightColors.bgSoft,
+  surfaceContainer: TokyoNightColors.bgElevated,
+  surfaceContainerHigh: TokyoNightColors.bgHighlight,
+  surfaceContainerHighest: TokyoNightColors.fgGutter,
+  onSurface: TokyoNightColors.fg,
+  onSurfaceVariant: TokyoNightColors.fgDark,
+  outline: TokyoNightColors.comment,
+  outlineVariant: TokyoNightColors.terminalBlack,
+
+  // Primary — Save button, selected nav indicators
+  primary: TokyoNightColors.blue,
+  onPrimary: TokyoNightColors.bgDark,
+  primaryContainer: TokyoNightColors.bgHighlight,
+  onPrimaryContainer: TokyoNightColors.fg,
+
+  // Secondary — Test button (not-passing state)
+  secondary: TokyoNightColors.magenta,
+  onSecondary: TokyoNightColors.bgDark,
+  secondaryContainer: TokyoNightColors.fgGutter,
+  onSecondaryContainer: TokyoNightColors.fg,
+
+  // Tertiary — swipe-to-unread background
+  tertiary: TokyoNightColors.teal,
+  onTertiary: TokyoNightColors.bgDark,
+  tertiaryContainer: TokyoNightColors.bgElevated,
+  onTertiaryContainer: TokyoNightColors.fg,
+
+  // Error
+  error: TokyoNightColors.red,
+  onError: TokyoNightColors.bgDark,
+  errorContainer: TokyoNightColors.brightRed,
+  onErrorContainer: TokyoNightColors.fg,
+
+  // Inverse / scrim
+  inverseSurface: TokyoNightColors.fg,
+  onInverseSurface: TokyoNightColors.bgDark,
+  inversePrimary: TokyoNightColors.blue,
+  scrim: TokyoNightColors.bgDarker,
+  shadow: TokyoNightColors.bgDarker,
+);
+
+final ThemeData tokyoNightDarkTheme = buildAppTheme(
+  colorScheme: _tokyoNightColorScheme,
+  appColors: const AppColorExtension(
+    success: TokyoNightColors.green,
+    onSuccess: TokyoNightColors.bgDark,
+    warning: TokyoNightColors.yellow,
+    onWarning: TokyoNightColors.bgDark,
+    info: TokyoNightColors.blue,
+    onInfo: TokyoNightColors.bgDark,
+  ),
+);