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/settings/settings.dart

@@ -292,14 +292,25 @@ class _SettingsState extends State<Settings> {
                 labelText: 'Theme',
               ),
               items: const [
+                DropdownMenuItem(value: 'alucard', child: Text('Alucard')),
+                DropdownMenuItem(
+                  value: 'catppuccin_latte',
+                  child: Text('Catppuccin Latte'),
+                ),
                 DropdownMenuItem(
                   value: 'catppuccin_mocha',
                   child: Text('Catppuccin Mocha'),
                 ),
+                DropdownMenuItem(value: 'dracula', child: Text('Dracula')),
                 DropdownMenuItem(
                   value: 'gruvbox_dark',
                   child: Text('Gruvbox Dark'),
                 ),
+                DropdownMenuItem(value: 'nord', child: Text('Nord')),
+                DropdownMenuItem(
+                  value: 'tokyo_night',
+                  child: Text('Tokyo Night'),
+                ),
               ],
               // Applies immediately; the theme persists itself, no Save needed.
               onChanged: (value) {