feat(android): Implement immersive mode for Flutter app

- Enable Flutter view to render behind system UI (status bar, navigation bar)
- Set transparent status bar and navigation bar colors in styles.xml
- Enable immersive sticky mode in Flutter main.dart
- MainActivity.kt: Set decor fits system windows to false for full-screen layout

android/app/src/main/java/com/tokilabs/mattermost/MainActivity.kt

@@ -1,6 +1,7 @@
 package com.tokilabs.mattermost
 
 import android.os.Bundle
+import androidx.core.view.WindowCompat
 import com.tokilabs.mattermost.helpers.DatabaseHelper
 import com.tokilabs.mattermost.helpers.Network
 import io.flutter.embedding.android.FlutterActivity
@@ -100,6 +101,8 @@ class MainActivity : FlutterActivity() {
 
     override fun onCreate(savedInstanceState: Bundle?) {
         super.onCreate(savedInstanceState)
+        WindowCompat.setDecorFitsSystemWindows(window, false)
+
         // 알림 탭으로 앱 진입 시 Intent 데이터를 Flutter로 전달
         intent?.extras?.let { extras ->
             val data = mutableMapOf<String, Any?>()

android/app/src/main/res/values-night/styles.xml

@@ -5,6 +5,8 @@
         <!-- Show a splash screen on the activity. Automatically removed when
              the Flutter engine draws its first frame -->
         <item name="android:windowBackground">@drawable/launch_background</item>
+        <item name="android:statusBarColor">@android:color/transparent</item>
+        <item name="android:navigationBarColor">@android:color/transparent</item>
     </style>
     <!-- Theme applied to the Android Window as soon as the process has started.
          This theme determines the color of the Android Window while your
@@ -14,5 +16,7 @@
          This Theme is only used starting with V2 of Flutter's Android embedding. -->
     <style name="NormalTheme" parent="@android:style/Theme.Black.NoTitleBar">
         <item name="android:windowBackground">?android:colorBackground</item>
+        <item name="android:statusBarColor">@android:color/transparent</item>
+        <item name="android:navigationBarColor">@android:color/transparent</item>
     </style>
 </resources>

android/app/src/main/res/values/styles.xml

@@ -5,6 +5,8 @@
         <!-- Show a splash screen on the activity. Automatically removed when
              the Flutter engine draws its first frame -->
         <item name="android:windowBackground">@drawable/launch_background</item>
+        <item name="android:statusBarColor">@android:color/transparent</item>
+        <item name="android:navigationBarColor">@android:color/transparent</item>
     </style>
     <!-- Theme applied to the Android Window as soon as the process has started.
          This theme determines the color of the Android Window while your
@@ -14,5 +16,7 @@
          This Theme is only used starting with V2 of Flutter's Android embedding. -->
     <style name="NormalTheme" parent="@android:style/Theme.Light.NoTitleBar">
         <item name="android:windowBackground">?android:colorBackground</item>
+        <item name="android:statusBarColor">@android:color/transparent</item>
+        <item name="android:navigationBarColor">@android:color/transparent</item>
     </style>
 </resources>

lib/main.dart

@@ -1,6 +1,7 @@
 import 'dart:async';
 import 'package:firebase_core/firebase_core.dart';
 import 'package:firebase_messaging/firebase_messaging.dart';
+import 'package:flutter/services.dart';
 import 'package:flutter/material.dart';
 import 'services/mattermost_auth_service.dart';
 import 'services/push_notification_background.dart';
@@ -9,9 +10,20 @@ import 'services/push_notification_service.dart';
 final _navigatorKey = GlobalKey<NavigatorState>();
 final _pushService = PushNotificationService();
 
+Future<void> _applyFullscreenMode() async {
+  await SystemChrome.setEnabledSystemUIMode(SystemUiMode.immersiveSticky);
+  SystemChrome.setSystemUIOverlayStyle(const SystemUiOverlayStyle(
+    statusBarColor: Colors.transparent,
+    systemNavigationBarColor: Colors.transparent,
+    systemNavigationBarDividerColor: Colors.transparent,
+  ));
+}
+
 void main() async {
   WidgetsFlutterBinding.ensureInitialized();
 
+  await _applyFullscreenMode();
+
   await Firebase.initializeApp();
 
   FirebaseMessaging.onBackgroundMessage(firebaseMessagingBackgroundHandler);
@@ -68,6 +80,9 @@ class _MyHomePageState extends State<MyHomePage> {
   @override
   void initState() {
     super.initState();
+    WidgetsBinding.instance.addPostFrameCallback((_) {
+      _applyFullscreenMode();
+    });
     _notifSub = _pushService.onNotification.listen((data) {
       if (data['type'] != 'message') return;
       final message = data['message'] as String? ?? '';
@@ -97,10 +112,6 @@ class _MyHomePageState extends State<MyHomePage> {
   @override
   Widget build(BuildContext context) {
     return Scaffold(
-      appBar: AppBar(
-        backgroundColor: Theme.of(context).colorScheme.inversePrimary,
-        title: Text(widget.title),
-      ),
       body: const Center(
         child: Text('NomadCode'),
       ),