Scaffold backend gateway and integration docs

lib/core/config/app_config.dart

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+/// Application-level runtime configuration.
+class AppConfig {
+  AppConfig._();
+
+  static const String _defaultBaseUrl = String.fromEnvironment(
+    'BACKEND_BASE_URL',
+    defaultValue: 'https://api.example.com',
+  );
+
+  static String? _baseUrlOverride;
+
+  /// Returns the configured backend base URL.
+  static String get backendBaseUrl {
+    final String? override = _baseUrlOverride;
+    if (override != null && override.trim().isNotEmpty) {
+      return override.trim();
+    }
+    return _defaultBaseUrl;
+  }
+
+  /// Enables fake backend implementation for local/offline development.
+  static bool get useFakeBackend =>
+      const bool.fromEnvironment('USE_FAKE_BACKEND', defaultValue: false);
+
+  /// Runtime override for backend URL (e.g. local settings screen).
+  static void overrideBackendBaseUrl(String? baseUrl) {
+    _baseUrlOverride = baseUrl;
+  }
+}

lib/core/config/app_theme.dart

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+import 'package:flutter/material.dart';
+
+/// Shared app theme, aligned with the visual tone of the reference templates.
+class AppTheme {
+  AppTheme._();
+
+  static const Color background = Color(0xFFEDF0F2);
+  static const Color surface = Color(0xFFFFFFFF);
+  static const Color primary = Color(0xFF253840);
+  static const Color accent = Color(0xFF00B6F0);
+
+  /// Builds the Material [ThemeData].
+  static ThemeData light() {
+    final ColorScheme colorScheme = ColorScheme.fromSeed(
+      seedColor: primary,
+      primary: primary,
+      secondary: accent,
+      surface: surface,
+      brightness: Brightness.light,
+    );
+
+    return ThemeData(
+      useMaterial3: true,
+      colorScheme: colorScheme,
+      scaffoldBackgroundColor: background,
+      appBarTheme: const AppBarTheme(
+        centerTitle: false,
+        backgroundColor: surface,
+        foregroundColor: primary,
+        elevation: 0,
+      ),
+    );
+  }
+}