feat(meezi_app): Meezi green theme + rich discovery API (Koja parity, code-only)

Head-start on the Koja-Flutter build while pub access is unavailable (pub.dev 403
under sanctions). NOT yet built/verified — needs `flutter create` + `pub get` once
package access is restored.

- core/theme/app_theme.dart: centralized MeeziTheme (brand green #0F6E56, Material 3,
  filled/outlined buttons, inputs), wired into main.dart (was a brown seed, no theme).
- public_api.dart: discover() gains the full filter set (themes/vibes/occasions/
  spaceFeatures/noise/priceTier/size/openNow) + discoverNearby/nlpParse/discoverTaxonomy,
  matching the web Koja's backend surface. Follows the existing dio pattern.

mobile/meezi_app/lib/main.dart

@@ -3,6 +3,7 @@ import 'package:flutter_localizations/flutter_localizations.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 
 import 'app/router.dart';
+import 'core/theme/app_theme.dart';
 
 void main() {
   runApp(const ProviderScope(child: MeeziApp()));
@@ -22,10 +23,9 @@ class MeeziApp extends StatelessWidget {
         GlobalWidgetsLocalizations.delegate,
         GlobalCupertinoLocalizations.delegate,
       ],
-      theme: ThemeData(
-        colorScheme: ColorScheme.fromSeed(seedColor: const Color(0xFF6B4F3A)),
-        useMaterial3: true,
-      ),
+      theme: MeeziTheme.light(),
+      darkTheme: MeeziTheme.dark(),
+      themeMode: ThemeMode.light,
       routerConfig: appRouter,
     );
   }