refactor: extract shared placeholder widget and fix lifecycle

- Create FeatureNotImplementedScreen reusable widget
- Simplify GasSensorScreen and DustSensorScreen to use shared widget
- Remove redundant didChangeDependencies calls
- Keep lifecycle logic only in initState
- Reduce code duplication by ~200 lines

Addresses mentor feedback on code quality and maintainability

Author: Irfan-del-droid

lib/view/widgets/feature_not_implemented_screen.dart

@@ -0,0 +1,123 @@
+import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
+import 'package:pslab/l10n/app_localizations.dart';
+import 'package:pslab/providers/locator.dart';
+import 'package:pslab/view/widgets/common_scaffold_widget.dart';
+
+/// A reusable placeholder widget for features that are not yet implemented.
+/// Used to provide consistent UX across multiple placeholder screens.
+class FeatureNotImplementedScreen extends StatefulWidget {
+  final String title;
+  final String description;
+  final IconData icon;
+  final Color accentColor;
+
+  const FeatureNotImplementedScreen({
+    super.key,
+    required this.title,
+    required this.description,
+    required this.icon,
+    required this.accentColor,
+  });
+
+  @override
+  State<FeatureNotImplementedScreen> createState() =>
+      _FeatureNotImplementedScreenState();
+}
+
+class _FeatureNotImplementedScreenState extends State<FeatureNotImplementedScreen> {
+  AppLocalizations appLocalizations = getIt.get<AppLocalizations>();
+
+  @override
+  void initState() {
+    super.initState();
+    _setPortraitOrientation();
+    SystemChrome.setEnabledSystemUIMode(SystemUiMode.edgeToEdge);
+  }
+
+  void _setPortraitOrientation() {
+    SystemChrome.setPreferredOrientations([
+      DeviceOrientation.portraitUp,
+      DeviceOrientation.portraitDown,
+    ]);
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return CommonScaffold(
+      title: widget.title,
+      body: Center(
+        child: Padding(
+          padding: const EdgeInsets.all(24.0),
+          child: Column(
+            mainAxisAlignment: MainAxisAlignment.center,
+            children: [
+              Icon(
+                widget.icon,
+                size: 120,
+                color: Colors.grey.shade400,
+              ),
+              const SizedBox(height: 32),
+              Text(
+                widget.title,
+                style: Theme.of(context).textTheme.headlineSmall?.copyWith(
+                      fontWeight: FontWeight.bold,
+                      color: Colors.black87,
+                    ),
+              ),
+              const SizedBox(height: 16),
+              Text(
+                appLocalizations.screenNotImplemented,
+                textAlign: TextAlign.center,
+                style: Theme.of(context).textTheme.bodyLarge?.copyWith(
+                      color: Colors.grey.shade600,
+                    ),
+              ),
+              const SizedBox(height: 24),
+              Container(
+                padding: const EdgeInsets.all(16),
+                decoration: BoxDecoration(
+                  color: widget.accentColor.withOpacity(0.1),
+                  borderRadius: BorderRadius.circular(8),
+                  border: Border.all(color: widget.accentColor.withOpacity(0.3)),
+                ),
+                child: Column(
+                  children: [
+                    Icon(
+                      Icons.info_outline,
+                      color: widget.accentColor,
+                      size: 32,
+                    ),
+                    const SizedBox(height: 8),
+                    Text(
+                      widget.description,
+                      textAlign: TextAlign.center,
+                      style: TextStyle(
+                        color: widget.accentColor,
+                        fontSize: 14,
+                      ),
+                    ),
+                  ],
+                ),
+              ),
+              const SizedBox(height: 32),
+              ElevatedButton.icon(
+                onPressed: () {
+                  Navigator.pop(context);
+                },
+                icon: const Icon(Icons.arrow_back),
+                label: Text(appLocalizations.close),
+                style: ElevatedButton.styleFrom(
+                  padding: const EdgeInsets.symmetric(
+                    horizontal: 24,
+                    vertical: 12,
+                  ),
+                ),
+              ),
+            ],
+          ),
+        ),
+      ),
+    );
+  }
+}